Assign AUX funtions to joystick buttons.

#### Issue details
at the moment its not possible to trigger aux functions from a joystick unless you assign the function to an analogue channel. There are very few options for the joystick buttons, they should be able to do anything you can do from the aux menu.

Start by tracing how Mission Planner handles joystick buttons and how functions are exposed through the AUX menu. Done means joystick buttons can trigger the same AUX functions currently available through that menu, without requiring an analogue channel.

Written by the indexing model from the issue text.