Plane: changed preflight calibration to prevent accel cal from MP

MissionPlanner sends param1=1 and param3=1 for preflight
calibration. This was having the effect of redoing the accel
calibration as 1D cal on every flight!
Rover: make preflight calibration follow plane conventions
Plane: prepare for 3.1.2beta3
build: added motor_test, and fixed build checks for PX4
AP_HAL: added set_esc_scaling() RCOutput function

this will be used to scale outputs for uavcan ESCs based on throttle
range
HAL_PX4: publish actuator values for UAVCAN ESCs

this publishes scaled actuator values so that the uavcan module can
drive ESCs
Rover: setup pwm esc scaling
Copter: setup pwm esc scaling
Plane: setup pwm esc scaling
DataFlash: added logging of ESC feedback (currently only available with UAVCAN ESCs)
DataFlash: log each ESC separately

this saves log space for ESCs that are not initialised (eg. only 3rd
ESC on fixed wing) and is a bit easier to deal with as a user
DataFlash: prevent build errors on non-PX4 targets