vehicle state.

refactor use_obstacle_avoidance method

acceptance radius from the paramters and overwrite it if the mission item
has set acceptance radius

The hover thrust param also changed the thrust scaling in Stabilized mode.
However if the hover thrust is very low (e.g. below 20%), the throttle
stick input becomes very distorted.

- no airframe changes the default
- it does not make much sense to be able to configure the 0.1 threshold
  but the 0.3 threshold for ground contact detection cannot be configured.

radio_status: change 'fixed' field name to 'fix' so it does not clash with 'fixed' floating-point notation on the IDL generation

The user does not need to configure this, so we can hide it from the UI.