HAL_PX4: switched scheduler to use a pthread

this allows the timer tasks to access file descriptors in the main APM
task, which makes writing PX4 device drivers much easier
HAL_PX4: added AnalogIn driver

this allows airspeed to work on a PX4
HAL_PX4: start adc driver in rc.APM
Plane: setup PX4 airspeed analog port
HAL_PX4: enabled AnalogIn driver
Plane: cleanup driver declaration

remove a lot of the #if nesting
Rover: fixed mavlink fetch of 16 character parameter
Copter: fixed mavlink fetch of 16 character parameter
Plane: fixed mavlink fetch of 16 character parameter
HAL_PX4: added Util instance

this fixes hal.util->vsnprintf_P()