[imu] body_to_imu: use generic orientation representation and expose it in settings to change during operation

A bit of cleanup, less copies of body_to_imu in other ahrs implementations.
And you can now change the body_to_imu orientation via settings from the GCS, which should make experimentally "tuning" this for nice hover easier.

I guess we could also remove the ins_roll_neutral and ins_pitch_neutral in all AHRS algos that still use AHRS_UPDATE_FW_ESTIMATOR.