DMA buffers now work reliably with USART. The DMA api has changed to make it
more efficient code-size-wise. This may, however, complicate things when
implementating powersaving features
The dma buffer should be services that are used on top of peripherals. As such,
the usart driver should'nt directly use them, this is up to the user. The
multi-buffer has also been simplified since I was not satisfied with the
previous implementation
Transfer Complete is preferable to TX Empty when using a DMA since we could be
polling the bit between transfers. TXE would be 1 but the DMA would be
transfering another at the same time. TC takes the DMA into account
