EKF2: allow sideslip fusion to always start with airspeed fusion

[bresch]

Solved Problem

During front transition, the drone is still considered as a multirotor. Airspeed fusion can start when there is enough wind-relative velocity but sideslip fusion is still strictly relying on the fixed-wing flag. This causes race conditions in commander when the transition to fixedwing occurs as the ekf needs to receive the message before activating the sideslip fusion.

Solution

The sideslip fusion parameter tells the EKF that this fusion type is possible on this vehicle. We know that it's only possible when airspeed is significant so we can let it start when the airspeed fusion starts as well (regardless of the vehicle type).
This also gives us a bit more flexibility as a multirotor with a vertical stabilizer and an airspeed sensor could also enable sideslip fusion.

Test coverage