Flow assistive LED lighting controller

This PR adds LED control through PWM based on PX4FLOW Quality factor.
The extras.txt file has to be added to the SD card to adjust min and max PWM values for the channel on which the LED is connected.
A custom mixer has to be added on the SD card in etc/mixers/[...].mix to change the channel that will be used to actuator control group 1, and scale output from 0 to 1. (Links to the files will be provided.)

PID parameters have been added to control the LED intensity with respect to FLOW Quality.

This is WIP.