rc_channels cleanup

[DrTon]

Now only topic cleanup, but I want also to optimize sensors app to avoid massive code duplication in RC channels handling. @LorenzMeier, any ideas how to do this better? Maybe move to separate app?

BTW, rc_channels topic is not used anywhere, only in sdlog2, but we can use rc_input or manual_control_setpoint here.

[LorenzMeier]

We should probably log more than 8 channels and we probably should limit ourselves to int16 to save bandwidth. I would propose this layout:
channel 1-14
channel count
rssi value

This would cover 90% of all setups. Most setups have more than 8 channels, but logging all e.g. 18 is usually also not necessary (because these are the mapped channels and 14 mapped channels is usable).

[LorenzMeier]

@DrTon Merging as is - regarding your question: If we would move this code into a library call, we could execute it directly in the IO and FMU drivers. But the rework here is already quite useful.