Debug logging of telemetry

[smig]

It might be useful for troubleshooting purposes, to provide the ability to configure RCP to write a debugging log to the SD card. In particular, logging the commands and responses between RCP and the Realtime telemetry module would allow folks to view the log for errors and anomalies. This might assist those who hit issue getting telemetry working.

I could also see this being applied to other areas/extension modules in the future.