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
HAL_Linux: Enable external clk for RCOutput_Navio
That should fix PWM value drifts.
HAL_Linux: Optimize bandwidth for RCOutput_Navio
40% less bytes in I2C transactions for PCA9685.