add param_map_rc message

@LorenzMeier 
This is a proposal for a message which ask the system to map/bind a rc channel to a parameter.

When the rc channel changes the system should update the parameter value. This feature allows to use a potentiometer knob for parameter tuning.

PR for px4 and qgc will follow.
@thomasgubler I don't get why this is needed? The parameter itself already has a default value as well as a whatever it's current value is. What does this do? Won't whatever this value is be immediately stomped by the actual RC input value?