AP_Frsky_Telem: added initial Frsky telemetry library

based on work from the PX4Firmware tree, this adds a library which can
create and send Frsky telemetry packets
Copter: Add SERIAL2_PROTOCOL for GCS and FRsky telemtry

this allows selection of protocol type on telem2. The default is
MAVLink, but can be selected as FrSky protocol
Plane: Add SERIAL2_PROTOCOL for GCS and FRSky telemtry

This allows selection of protocol type on telem2. The default is MAVLink, but can be selected as FrSky protocol
Rover: Add SERIAL2_PROTOCOL for GCS and FRSky telemtry

This allows selection of protocol type on telem2. The default is MAVLink, but can be selected as FrSky protocol