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.
the param value as a function of rc input is

```
param_val = constrain(value0 + rc * scale, min, max)
```

The value0 field allows the user to set a new centerpoint. For instance if the current value is 0.5 you can tune around 1.0 if you like.