AP_Math: added a quick test of floating point location accuracy

just confirming some maths ...
PX4: fixed airspeed build on FMUv1
AP_HAL: added RCOutput example sketch

useful for testing all 14 channels via APM API
HAL_PX4: fixed GPIO build

don't use vehicle includes
PX4: disable mixer load for now

loading a mixer disables IO output
PX4: expand pass thru mixer to 8 channels

we're not using it now, but when we do use it we want all 8 channels