Wake up from UART/serial activity

[microbit-carlos]

So that the MicroPython REPL, one of the main way for PC tools to interact with MicroPython, can still work.

As discussed in:

• Wake up from power.deep_sleep() via UART incoming data? micropython-microbit-v2#127

And implemented in:

• Merging bug/deepsleep-anyway, feature/serial-wake and the hotfix all back into master lancaster-university/codal-microbit-v2#229