[agl] use ground_alt from autopilot if no SRTM data available
[agl] fix waypoint edit box display and sim ground detect
[srtm] improving information messages about SRTM data
[ground_alt] remove ground_alt from rotorcraft firmware

use directly state interface ref alt

ground_alt and state ref was getting out of sync when calling geo_init
only one time, making issues with nps when setting initial position far
from the flight plan ref
[ground_alt] add GetAltRef() to FW nav and update flight plans
Merge pull request #714

For displaying AGL in GCS: use ground_alt from autopilot if no SRTM data available.

Please replace ground_alt with GetAltRef() in your fligh plans.

* nav_ref:
  [ground_alt] add GetAltRef() to FW nav and update flight plans
  [ground_alt] remove ground_alt from rotorcraft firmware
  [srtm] improving information messages about SRTM data
  [agl] fix waypoint edit box display and sim ground detect
  [agl] use ground_alt from autopilot if no SRTM data available