Arduplane: add support for simple digital pin.

[taedla01]

For future developments, add support for simple digital pins on FMU rail
like ArduRover.

[gmorph]

We tend not to add code that we aren't going to use. It looks like a cut and paste of the Rover method. Is there any reason for adding this code now if its not going to be used?
Thanks, Grant.

[taedla01]

Hello everyone,

@gmorph Sorry... magicrub commented on other PR that this is dead code. Yes, it is a cut and paste from Rover. I saw this on current ArduRover code and thought that was being still used.

We are using this small piece of code for multiple uses:
1- With a switch input on pin 55, we can change flight batteries without the needed of turning off the plane. Pushing the switch, Pixhawk Enters on a "standby mode", activating a custom made capacitor circuit, giving power for at Least 40 seconds.
2- Connecting the logical level camera flash hot shoe pin to pin 54 gives (with other code changes) a best camera triggering flag. Reducing the CAM messages lag from 200ms to at Least 20-30ms.
3-With another switch we turn on/off the flight lights.

Thought that others could use for simplest switch readings...

Cheers,
Dario

[taedla01]

Hello everyone,
Included with #2309, so will close.