HAL_PX4: added automatic hardware flow control detection

this allows us to detect if hardware flow control is not available and
automatically disable it
AP_BoardConfig: set flow control to AUTO by default
AP_BoardConfig: PX4v1 does not have hw flow control on UART5
Coax: Yaw fix

Was using _rc_yaw servo_out instead of pwm_out to modify motors.
Corrected and less touchy in Yaw
Copter: only report gps glitch when usb disconnected
AP_HAL: added force_safety_off rcoutput function

this forces the safety switch off, enabling PWM on the IO board
AP_BoardConfig: added BRD_SAFETYENABLE option

this allows the safety switch to be disabled on startup
HAL_PX4: remove an old conditional define