Frsky bidirectional telemetry implementation

[tridge]

I’ve merged in FPort support in master. Please see this post for details:

FrSky FPort support - testers wanted

[serveimage] I’ve just merged support for the FrSky FPort RC protocol into master. I’d like to get some feedback from people willing to test it. You can read more about FPort here: The support has been tested on a X4R-SB receiver with the beta firmware loaded for FPort support. I have tested it on several boards and it is working for me, but I’d like a wider range of testers. There are two aspects to FPort support. The first is support for the incoming RC protocol. That should work on al…

Note that the changes for FPort support should also help a lot of people using SPort. We have new BRD_ALT_CONFIG options, plus I have improved the handling of half-duplex mode to work on a wider variety of boards.