[riot-notifications] [RIOT] cpu/samd21: optimized and extended UART impl. (#3838)

[Hauke Petersen]

This is needed for a boarder router example using the samr21-xpro:

I optimized the samd21's UART driver quite a bit and defined a second UART interface for the samr21-xpro board (RX: PA23, TX: PA22) -> also documented [here](https://docs.google.com/spreadsheets/d/1350pEm6-6vR7L_JGR5rHhaNbMrZgzkCgA-OS0akPH6U/edit#gid=1766063117)

This PR also saves quite some memory:
```
examples/default - master:
   text	   data	    bss	    dec	    hex
  14664	    156	   2884	  17704	   4528	

examples/default - this branch:
   text	   data	    bss	    dec	    hex
  14360	    156	   2740	  17256	   4368
```
-> Saves 304 byte ROM. I am not sure though, where the 144 byte of RAM savings are coming from...
You can view, comment on, or merge this pull request online at:

  https://github.com/RIOT-OS/RIOT/pull/3838

-- Commit Summary --

  * cpu/samd21: optimized UART implementation
  * boards/samr21-xpro: adapted and extended UART defs

-- File Changes --

    M boards/samr21-xpro/include/periph_conf.h (24)
    M cpu/samd21/include/periph_cpu.h (23)
    M cpu/samd21/periph/uart.c (242)

-- Patch Links --

https://github.com/RIOT-OS/RIOT/pull/3838.patch
https://github.com/RIOT-OS/RIOT/pull/3838.diff

---
Reply to this email directly or view it on GitHub:
https://github.com/RIOT-OS/RIOT/pull/3838
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