[rotorcraft] separate KILL_ON_GROUND_DETECT with higher default threshold
- if KILL_ON_GROUND_DETECT is TRUE, enable killing of motors on ground detection
  used in default land block, add to your airframe file:
  <define name="KILL_ON_GROUND_DETECT" value="TRUE"/>
- THRESHOLD_GROUND_DETECT is now in m/s^2 and defaults to 25 m/s^2