Add an option to use a compass for pre-arm consistency check but not for heading estimation

### Feature request

**Is your feature request related to a problem? Please describe.**
The Cube is mounted close to a battery and sees nearly 2000 mGa field intensity when the vehicle's propulsion is operating. An external magnetometer is in use for heading estimation. I would like the magnetometer consistency check to check the external mag against the internal mag prior to arming the vehicle.

**Describe the solution you'd like**
Perhaps COMPASS_USE could have a new option 2, where the compass is used for consistency checks, but not for heading determination.

**Platform**
[ x ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Start by tracing the COMPASS_USE parameter and the existing compass consistency and heading-estimation paths. The work is complete when a new option performs the external-versus-internal magnetometer consistency check before arming while excluding that compass from heading determination; validate both behaviors in the relevant vehicle scenarios.

Written by the indexing model from the issue text.