UART library to support combinable Rx tasks

[ed-xmos]

Currently each UART instance requires a single xcore thread per Rx (Tx is handled inline, although could be offloaded to a thread). To support multiple Rxs in a single xcore thread (and possibly even a buffered Tx) we would need a manually developed select based UART (similar to XC's combinable feature). It may also be possible using this technique to turn the Tx into a buffered version sharing the Rx xcore thread which would allow it to behave like a typical UART HW peripheral.