Autopilot in HUD always stays unknown

#### Issue details
What Mavlink message (and what parameter) should be sent to MP to modify the autopilot field in the HUD?

#### Version
latest

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[x] Plane
[ ] Rover
[ ] Sub

#### Airframe type
fixed wing

#### Hardware type
custom Raspberry PI based FCC

Start with the Plane HUD's autopilot field and trace how it interprets incoming MAVLink messages and parameters. Determine which message and parameter should provide the autopilot value, then verify that the HUD no longer displays Unknown for the reported Raspberry Pi-based Plane setup.

Written by the indexing model from the issue text.