- set MAV_TYPE as a parameter default per vehicle type, or airframe if necessary
- cleanup MAV_TYPE param metadata and commander helper to only include
what's currently used in PX4
* add support for unmanned underwater vehicles:
* airframe uuv_generic + uuv_hippocammpus including mav_type = 12 for submarines
* mixer for UUVs with X-shaped thruster setup similar to quadcopter
* add module uuv_att_control for underwater robot attitude control
* add rc.uuv_defaults/apps for autostarting e.g. ekf2 and uuv_att_control app