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'm not quite sure if this is in the xml but I had talked to Lorenz about this previously: The param_id is a char[16], but the function signature ends up having it as a char * instead of a char[16]. This causes confusion on the callers to the functions with this signature. The reason being that the mavlink code that is generated does a memcpy of the full 16 bytes. If you call it accidentally without the full 16 bytes of characters you can run into an intermittent, hard to track down crash in QGC since you walk off the end of the array with the memcpy. For example your QGC code you just put up a pull for has this exact bug in it. I don't know if there is a different way to define the field to get this signature to work correctly or if it is a mavlink code generator problems. With the right signature the QGC compile would fail on this bug.