Add IRQ-driven buffered USART implementation for STM32 v2 usart (#356)

* Add IRQ-driven buffered USART implementation for STM32 v2 usart

* Implementation based on nRF UARTE, but simplified to not use DMA to
  avoid complex interaction between DMA and USART.
* Implementation of AsyncBufRead and AsyncWrite traits
* Some unit tests to ring buffer
* Update polyfill version
* Update sub module to get usart IRQ fix

1 files changed, 0 insertions, 0 deletions

diff --git a/stm32-data b/stm32-data
-Subproject af0611ff9b29f2c0ab675a4f0e51d8241a4097f
+Subproject 79ab92a38d1cbb47dda2f6f8556fe1abc9a14f3