mc_pos_control_main smooth takeoff: enable position-xy/yaw control once above 0.5m from takeoff reference

just keep thrust vector pointing upward

smooth-takeoff. This is only required if someone writes a FlightTask where both position
and velocity setpoints are finite

made changes based on reviewer feedback. Also added a bool option for block_read and block_write PEC, as some devices may not use the PEC

    - libuavcan in PX4/Firmware (8958b42dada1e14f27c287a59b51ff26e90486a1): https://github.com/UAVCAN/libuavcan/commit/479d144ded9d5d16e234121ef9511b9ca9dfdac5
    - libuavcan current upstream: https://github.com/UAVCAN/libuavcan/commit/6174b8c10a2dbf47076ca7a7b5820a4c36c8a988
    - Changes: https://github.com/UAVCAN/libuavcan/compare/479d144ded9d5d16e234121ef9511b9ca9dfdac5...6174b8c10a2dbf47076ca7a7b5820a4c36c8a988

    6174b8c 2018-10-06 Pavel Kirienko - Link to the new forum

-Rearranged bus options to match other drivers
-Added TODO statement because driver cannot yet start on a specific external bus

- Added two external I2C bus options corresponding to newer Pixhawk hardware (px4fmu-v5)
- Switched from getopt to px4_getopt
- Fixed bus checking loop exiting early

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.

Clarify which  componets is on which index for the item array