Add additional telemetry mode for improved mavlink support. Use modified...

... version of FastSerial to replace stock arduino serial port.

Hi, This is quite a large pull-request, but I have really tried to make it as clean as possible...
It will add another telemetry mode (mavlink) in which one can configure the telemetry downlink packet size and also mavlink radio status injection packets to let the APM (autopilot) adjust it's rate of which it sends telemetry through the openLRSngRX to the groundstation (openLRSngTX).
I've tried getting the job done with arduino's serial implementation, but didn't get it to work as good as with the FastSerial library. So I modified this library slightly so that it uses no dynamic memory allocation and tried to slim it down in size. It should fit for all currently available hardware configurations and I've also verified that it compiles ok under linux with the Makefile (allfw target).

If this change is too intrusive, just reject it, but I think many people out there can benefit from this feature.
Any questions, just send me an e-mail or discuss here (Im open for suggestions etc).

/Br