AP_HAL_PX4: use rc status flags and channel count for a more robust rc loss/failsafe handling
AP_HAL_PX4: require at least 5 channels for a valid RC signal
Merge remote-tracking branch '3dr/master' into explicit_failsafe3
Merge branch 'explicit_failsafe3' into failsafe_tones2

Conflicts:
	libraries/AP_HAL_PX4/RCInput.cpp