[rotorcraft] improve in_flight detection heuristic

Simply checks if thrust, speed and acceleration are above a threshold.
Does not rely on RC thrust command anymore, but only on actual thrust command.
Not tested on real vehicle at all so far...

Attempt to improve issue #201 and replaces #468
[rotorcraft][guidance] fixes for vertical nav
[rotorcraft] fix takeoff-land-takeoff in nav via flightplan
[rotorcraft] AUTOPILOT_IN_FLIGHT_MIN_THRUST defaults to 500 (~5%)
Merge pull request #469 from flixr/in_flight_heuristic

[rotorcraft] improve in_flight detection heuristic and fix takeoff-land-takeoff in nav