AHRS/EKF attitude at startup incorrect when AHRS_ORIENTATION not 0

[rmackay9]

This is re-raising an issue reported during 4.6 beta testing but it has existed for some time.

If the vehicle is placed upside down and AHRS_ORIENTATION = 8 (Roll 180) then the vehicle's attitude shown on the GCS HUD is incorrect for the first few seconds (at least). It then rolls or pitches towards the correct attitude and then suddenly corrects itself. It would be nice if the attitude was close to correct immediately upon startup.

This is an example of the behaviour captured by setting LOG_DISARMED = 1 and restarted the autopilot in an inverted position (and AHRS_ORIENTATION = 8)

The result is even more stark on Plane perhaps because it tends to use DCM soon after startup

4.6 PR that attempted to fix the issue: #29667

[LzREngineering]

Can confirm I am seeing same issue on a plane with Matek H743 v2 installed upside-down (orientation 12) - this was the same on all beta versions of 4.6 (I did not install any previous AP versions on this plane)