vmount: refactor to auto-select between all inputs

It is not convenient having to change a parameter to change a gimbal
from RC input to mavlink input mode or back. This refactor changes the
behaviour to use whatever is available, RC or mavlink commands.

Once a mavlink command is followed, control can be taken back using RC,
however, this requires a clear stick change.

src/drivers/vmount/input_test.cpp

@@ -1,6 +1,6 @@
 /****************************************************************************
 *
-*   Copyright (c) 2016 PX4 Development Team. All rights reserved.
+*   Copyright (c) 2016-2017 PX4 Development Team. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
@@ -57,7 +57,7 @@ bool InputTest::finished()
 	return true; /* only a single-shot test (for now) */
 }
 
-int InputTest::update(unsigned int timeout_ms, ControlData **control_data)
+int InputTest::update(unsigned int timeout_ms, ControlData **control_data, bool already_active)
 {
 	//we directly override the update() here, since we don't need the initialization from the base class