Add Mount option to retract on landing

[peterbarker]

[amilcarlucas]

@BITMASK metadata documentation is missing

[peterbarker]

@bitmask metadata documentation is missing

Fixed, thanks!

[nexton-winjeel]

How urgently do you want this capability in? We've got a similar feature that is vehicle-agnostic that:

• uses the AHRS to get height above ground (falling back to height above home)
• will auto-deploy when altitude goes above parameterised DEPLOY_ALT
• will auto-retract when altitude falls below parameterised RETRACT_ALT

Haven't pushed a PR for it yet because we never flight tested it enough (but we do have SITL tests).