DataFlash: added Log_Write_Message()

useful for logging general messages
Rover: send all mavlink string messages to flash log
Rover: make AUTO_TRIGGER_PIN be a push on/push off
AP_HAL: added set_stop_pin() and set_settle_time() interfaces to analogin

this is for multiple sonars on one vehicle, to prevent them
interfering with each other
HAL_AVR: support set_stop_pin() and set_settle_time() for analogin

useful for dual sonar support
HAL_AVR: initialise settle time
HAL_Empty: add new API functions
AP_Rangefinder_analog: implement settle time and stop_pin
Rover: don't do failsafe if throttle is below failsafe level
SITL: fixed build with new AnalogIn API
HAL_PX4: fixed build with new AnalogIn API

pin settle times not actually implemented on PX4 yet
HAL_SMACCM: fixed build for new AnalogIn API
AP_RangeFinder: removed special case for APM1

the airspeed port doesn't have enough power for the sonar anyway
AP_ADC: added dummy stop_pin and settle_time calls