leds_on_a_switch.lua: change LED brightness using an RC switch

ArduPilot controls LED brightness via the parameter NTF_LED_BRIGHT. This script allows you to control the value of the parameter using an RC switch.

Configure an RCx_OPTION as 300 for scripting and load this script. The low position will turn off the LEDs, the high position with give maximum brightness and the middle position will dim the LEDs