Pixhawk RSSI setup with TBS Crossfire (Pixhawk V3.4 firmware required)

How to setup the Pixhawk and TBS crossfire for RSSI input. This shows how to send RSSI across a PPM stream into the Pixhawk, First we setup the TBS crossfire to produce LQ (Link quality - like RSSI) into the channel 12 on the PPM stream Then we tell the Pixhawk which channle to use and also the min/maximum limits.