RFM22b TauLink serial bridge

[peabody124]

It would be very useful to have TauLink support a transparent
serial bridge (e.g. using VCP on the ground side) and a PIOS_COM
interface on the FC side that can be assigned like a normal
serial port.

This would be useful for things like a ROS serial link to be able
to talk through this link e.g. #1408

[peabody124]

Ok. I think this won't be very hard to implement. Basically I will replace this
https://github.com/TauLabs/TauLabs/blob/next/flight/PiOS/Common/pios_hal.c#L767-L784

Which assigns the RFM22b COM interface to the rf_telemetry ID. In RFM22B_TELEM modes it will have a RFM22B_SERIAL PIOS_HAL assignment that we can assign them in the same way we do other serial ports using PIOS_HAL_ConfigureCom.