<h3>Contribution description</h3>
<p>This PR removes the non-standard UART mode feature, and replaces it with the <code>uart_mode</code> implementation for both UART and LEUART (only supports 8 or 9 bit frames).</p>
<h3>Testing procedure</h3>
<p>Run <code>tests/periph_uart_mode</code> and follow the test instructions. All other tests that output to the shell should still work.</p>
<p>Here are some oscilloscope screenshots that show that the API is working (it's time-consuming to test this). I've used a custom application that just sends out 0x55.</p>
<p>UART 5N1:<br>
<a target="_blank" rel="noopener noreferrer" href="https://user-images.githubusercontent.com/815976/65815071-a5b1ca80-e1ea-11e9-874c-b24c5db6f3f4.png"><img src="https://user-images.githubusercontent.com/815976/65815071-a5b1ca80-e1ea-11e9-874c-b24c5db6f3f4.png" alt="uart-5n1" style="max-width:100%;"></a></p>
<p>UART 7E2:<br>
<a target="_blank" rel="noopener noreferrer" href="https://user-images.githubusercontent.com/815976/65815073-a64a6100-e1ea-11e9-82a0-6be38c25e997.png"><img src="https://user-images.githubusercontent.com/815976/65815073-a64a6100-e1ea-11e9-82a0-6be38c25e997.png" alt="uart-7e2" style="max-width:100%;"></a></p>
<p>LEUART 8N1 (default)<br>
<a target="_blank" rel="noopener noreferrer" href="https://user-images.githubusercontent.com/815976/65815095-dd207700-e1ea-11e9-9d83-235db1f74d1b.png"><img src="https://user-images.githubusercontent.com/815976/65815095-dd207700-e1ea-11e9-9d83-235db1f74d1b.png" alt="leuart-8n1" style="max-width:100%;"></a></p>
<p>LEUART 8O2:<br>
<a target="_blank" rel="noopener noreferrer" href="https://user-images.githubusercontent.com/815976/65815096-dd207700-e1ea-11e9-8ea3-be1c793b7dd1.png"><img src="https://user-images.githubusercontent.com/815976/65815096-dd207700-e1ea-11e9-8ea3-be1c793b7dd1.png" alt="leuart-8o2" style="max-width:100%;"></a></p>
<p>My scope show question marks whenever it cannot match the decoding settings to the actual data:</p>
<p><a target="_blank" rel="noopener noreferrer" href="https://user-images.githubusercontent.com/815976/65815072-a64a6100-e1ea-11e9-8437-4741c42305ed.png"><img src="https://user-images.githubusercontent.com/815976/65815072-a64a6100-e1ea-11e9-8437-4741c42305ed.png" alt="uart-7e2-questionmark" style="max-width:100%;"></a></p>
<h3>Issues/PRs references</h3>
<p>Depends on <a class="issue-link js-issue-link" data-error-text="Failed to load issue title" data-id="499750650" data-permission-text="Issue title is private" data-url="https://github.com/RIOT-OS/RIOT/issues/12322" data-hovercard-type="pull_request" data-hovercard-url="/RIOT-OS/RIOT/pull/12322/hovercard" href="https://github.com/RIOT-OS/RIOT/pull/12322">#12322</a>.</p>

<hr>

<h4>You can view, comment on, or merge this pull request online at:</h4>
<p>  <a href='https://github.com/RIOT-OS/RIOT/pull/12323'>https://github.com/RIOT-OS/RIOT/pull/12323</a></p>

<h4>Commit Summary</h4>
<ul>
  <li>cpu: efm32: move LOW_POWER_ENABLED to efm32-features.mk</li>
  <li>tests/cpu_efm32_features: update test</li>
  <li>boards+efm32: remove LOW_POWER_ENABLED from doc.txt</li>
  <li>efm32: remove UART modes via non-standard api</li>
  <li>pkg/gecko_sdk: update to newer version</li>
  <li>cpu/efm32: implement uart_mode</li>
  <li>boards+efm32: remove non-standard uart modes feature</li>
  <li>boards+efm32: provide periph_uart_modecfg</li>
  <li>tests/cpu_efm32_features: update test</li>
</ul>

<h4>File Changes</h4>
<ul>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-0">boards/ikea-tradfri/Makefile.features</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-1">boards/ikea-tradfri/include/periph_conf.h</a>
    (3)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-2">boards/slstk3401a/Makefile.features</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-3">boards/slstk3401a/doc.txt</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-4">boards/slstk3401a/include/periph_conf.h</a>
    (9)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-5">boards/slstk3402a/Makefile.features</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-6">boards/slstk3402a/doc.txt</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-7">boards/slstk3402a/include/periph_conf.h</a>
    (6)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-8">boards/sltb001a/Makefile.features</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-9">boards/sltb001a/doc.txt</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-10">boards/sltb001a/include/periph_conf.h</a>
    (9)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-11">boards/slwstk6000b/Makefile.features</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-12">boards/slwstk6000b/include/periph_conf.h</a>
    (3)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-13">boards/stk3600/Makefile.features</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-14">boards/stk3600/include/periph_conf.h</a>
    (9)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-15">boards/stk3700/Makefile.features</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-16">boards/stk3700/include/periph_conf.h</a>
    (9)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-17">cpu/efm32/Makefile.features</a>
    (4)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-18">cpu/efm32/doc.txt</a>
    (7)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-19">cpu/efm32/efm32-features.mk</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-20">cpu/efm32/include/periph_cpu.h</a>
    (52)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-21">cpu/efm32/periph/uart.c</a>
    (60)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-22">pkg/gecko_sdk/Makefile</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-23">tests/cpu_efm32_features/Makefile</a>
    (2)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-24">tests/cpu_efm32_features/README.md</a>
    (3)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12323/files#diff-25">tests/cpu_efm32_features/main.c</a>
    (22)
  </li>
</ul>

<h4>Patch Links:</h4>
<ul>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/12323.patch'>https://github.com/RIOT-OS/RIOT/pull/12323.patch</a></li>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/12323.diff'>https://github.com/RIOT-OS/RIOT/pull/12323.diff</a></li>
</ul>

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