Rover: GPS glitch can cause EKF failsafe

[rmackay9]

It seems that Rover will more easily enter an EKF failsafe compared with Copter. This can be easily reproduced in SITL by doing the following:

1. start Rover using "Tools/autotest/sim_vehicle.py --map --console -f motorboat"
2. create a small auto mission (e.g. a square)
3. auto, arm throttle
4. param set SIM_GPS1_GLTCH_X 0.0001

When the lane switch occurs (which also happens with Copter) the vehicle will immediately switch to Hold mode

This was found during testing of this EKF reset PR #31357

This issue is present in 4.6 and master/latest

[rmackay9]

There are two possible solutions to this:

1. set AHRS_OPTIONS = 3 to disable the fallback to DCM
2. change the EKF failsafe code in Rover so that it attempts to switch lanes before triggering. This bit of code exists in the Copter EKF failsafe but not Rover

[rmackay9]

This is resolved in 4.7 by disabling DCM by default