Please add support for RealFlight Interlink Elite Controller

Purchased RealFlight 7.5 just to get the Interlink controller to do functional testing of Arducopter. Currently it works as a Joystick, and the analog inputs for Ch1-4, and the Ch6 knob work as expected.

However, the Ch 5, 7 and 8 switches do not work as analog inputs, they are joystick "buttons".

Through testing, I have determined Ch5 is button input 0, down is off, up is on. It would be great if we could slave this artificially to the Ch5 input. When button 0 is off, Ch5 PWM is 1000, when it is on, Ch5 goes to 2000.

Ch 7 uses button input 0, down is off, up is on. Requires same artificial PWM control.

Ch8, uses 2 button inputs. "Down" turns Button 4 input on. "Middle turns Button 3 and 4 off. "Up" turns Button 3 on.

Start by locating Mission Planner's joystick input handling and reproduce the RealFlight Interlink Elite behavior described in the issue. Verify the existing analog channels, then implement and test the specified button-to-PWM mappings for channels 5, 7, and 8.

Written by the indexing model from the issue text.