- Move common code for USART (shared SPI / UART peripheral) to its
own file and allow sharing the USART peripheral to provide both
UART and SPI in round-robin fashion.
- Configure both UART and SPI bus via a `struct` in the board's
`periph_conf.h`
- this allows allocating the two UARTs as needed by the use case
- since both USARTs signals have a fixed connection to a single
GPIO, most configuration is moved to the CPU
- the board now only needs to decide which bus is provided by
which USART
Note: Sharing an USART used as UART requires cooperation from the app:
- If the UART is used in TX-only mode (no RX callback), the driver
will release the USART while not sending
- If the UART is used to also receive, the application needs to power
the UART down while not expecting something to send. An
`spi_acquire()` will be blocked while the UART is powered up.
