If S.Port is connected via external inverter or an uninverted signal is
used, the UART needs to be put into half-duplex mode.
This can be used to get uninverted S.Port:
https://oscarliang.com/uninverted-sbus-smart-port-frsky-receivers/
It is not needed for the Pixracer FrSky port.
* frsky_telemetry added support for hw flow control com port and minor improvements
* DTYPE tested OK. Return 0 for unix compatibility, whitespace removed. Full test start - status- stop - start -status OK