AP_Relay: add -1:Disabled to list of param values
Rover: remove unused RELAY_PIN definition

This is now part of the AP_Relay library
Copter: add Pixhawk pin to RSSI_PIN's @Value

Also prepend APM2 to existing definitions
Rover: add Pixhawk pin to RSSI_PIN's @Value

Also prepended APM2 to existing values.  Added A2 to list to be
consistent with plane and copter
Plane: prepended APM to RSSI_PIN's @Values

This makes it consistent with rover and copter and clarifies where the
pin is on the Pixhawk