Auto Mode with PlayStation controller (or other joysticks)

#### Issue details
When controlling the Pixhawk with a joystick, in my case a PS4 controller (I got the idea from BlueRobotics ROVs), there is no way that MP can execute Auto missions. When configuring the joystick, you must assign a button or stick to a function, such as left joystick controlling roll when moved side to side. In this case, the RC1 input maps to Roll. When assigning servo outputs, the only way to achieve manual control is to assign the desired servo output channel to RC1. However, for Auto to work, the servo output must be assigned to GroundSteering (as well as another servo output assigned to Throttle). So in order to change from manual to Auto, one must re-assign the servo outputs. I am thinking that a developer could internally map GroundSteering to RC1 and Throttle to RC3 so that calling GroundSteering also calls RC1, as manual mode would.

#### Version
Version 1.3.65 build 1.3.6672.30243

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[X] Rover (Using Boat Mode)
[ ] Sub

#### Airframe type
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_

#### Hardware type
Pixhawk 1

#### Logs
_Please provide a link to any relevant logs that show the issue_

The issue names no files or tests; start by tracing Mission Planner's joystick configuration and Rover/Boat Mode servo-output handling. Reproduce the manual-to-Auto transition with RC1/RC3 and GroundSteering/Throttle assignments, and consider the work done only when both modes operate without reassigning servo outputs.

Written by the indexing model from the issue text.