Add new posix_sitl_shell build target which allows to run -make posix_sitl_shell none- in order to get an empty shell

The SENS_FLOW_ROT parameter is used in the px4flow and mavlink receiver to perform the rotation from sensor frame to body frame.

The driver performs the rotation from sensor frame to body frame.
The recommended installation default is with the Y sensor axis pointing forward.

The sensors app assumed that all topics are published on boot which is not necessarily true and it assumed that all publications had valid data. This change ensures that topics are initialized as they update the first time and that the consistency check only runs on topics which carry valid data.

While at it, fix the trailing continuation line in the last item.

Without this fix, the function "get_time_inside" from navigation.h looses its purpose
to distinguish between takeoff and other waypoints.

- weathervane on takeoff
- separate cruising speeds for VTOL in MC and FW
- cruising speed resets
- mission work item logic is more clear
- fixed double execution of certain work item states
- enable cruise speed change on the fly by command
- moved VTOL transition target position generation to mission code and set always