<h3>Contribution description</h3>

<p>This PR adds riotboot support or nucleo-l476rg. It should work for all nucleo stm32l4 based boards.</p>
<h3>Testing procedure</h3>

<p>Run riotboot tests and everything should work as expected.</p>
<p><code>BOARD=nucleo-l476rg FEATURES_REQUIRED+=riotboot make -C tests/xtimer_usleep riotboot/flash-extended-slot0 term</code></p>
<p><code>BOARD=nucleo-l476rg  FEATURES_REQUIRED+=riotboot make -C tests/xtimer_usleep riotboot/flash-slot1 term</code></p>
<h3>Issues/PRs references</h3>

<p>Related to <a class="issue-link js-issue-link" data-error-text="Failed to load issue title" data-id="452926116" data-permission-text="Issue title is private" data-url="https://github.com/RIOT-OS/RIOT/issues/11641" data-hovercard-type="pull_request" data-hovercard-url="/RIOT-OS/RIOT/pull/11641/hovercard" href="https://github.com/RIOT-OS/RIOT/pull/11641">#11641</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/11643'>https://github.com/RIOT-OS/RIOT/pull/11643</a></p>

<h4>Commit Summary</h4>
<ul>
  <li>cpu/stm32l4: add CPU_FLASH_BASE</li>
  <li>stm32l4/Makefile.include: set RIOTBOOT_HDR_LEN to 0x200</li>
  <li>boards/nucleo-l476rg: add riotboot support</li>
</ul>

<h4>File Changes</h4>
<ul>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/11643/files#diff-0">boards/nucleo-l476rg/Makefile.features</a>
    (3)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/11643/files#diff-1">cpu/stm32l4/Makefile.include</a>
    (6)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/11643/files#diff-2">cpu/stm32l4/include/cpu_conf.h</a>
    (1)
  </li>
</ul>

<h4>Patch Links:</h4>
<ul>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/11643.patch'>https://github.com/RIOT-OS/RIOT/pull/11643.patch</a></li>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/11643.diff'>https://github.com/RIOT-OS/RIOT/pull/11643.diff</a></li>
</ul>

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