The "specific" lines configuration would not work due to index error in
the callback configuration. While fixing the error, simplifying the API
by moving the afio calls to the calling context seemed a cleaner way to
do things
A while back, macros had to be put in place to avoid letting the
compiler directly use the bitfields. This was necessary because the
compiler used strb instruction which only write bytes. On the AHB bus,
byte writes are transformed into word writes by repeating the byte,
which caused mayhem in the registers. After a lot of research, turns out
the packed attribute stops the compiler from does optimal (word) writes
and isn't needed anyway. Removing them fixes the issue
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
