This changes the way RC is handled for the Mantis:
- The RC values are re-written when arriving over MAVLink. They are
rescaled from 0..4095 to 1000..2000 and the channel bits added to
separate channels. This makes the downstream handling easier.
- Gimbal pitch is moved from Aux1 to Aux2 as that should be the default.
- Aux3 and Aux4 are used for the photo and video trigger.
- The speed button is used as the FLTMODE channel and set to switch
between POSCTL and ALTCTL.