Adds a communications backend based on the asynchronous UART API,
instead of the interrupt-driven UART API. The primary advantage of this
backend is an improved robustness to dropping bytes under high interrupt
or critical section loads.
Under all loads system efficiency is improved by:
* Reducing the time spent writing out individual bytes.
* Reducing the number of UART interrupts fired.
* Waking up the RX thread much less often.
When utilising this backend over `nordic,nrf-uarte` on a nRF52840, the
baudrate of an esp-at modem could be pushed to at least 921600 without
dropping bytes, compared to a maximum of 230400 with the interrupt API.
Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>
95d8deb572