flight termination on geofence violation
 create empty datalinkloss class for OBC

 Currently the class does the same as the RTL class. It is now possible
 whichclass is sued in the navigator to handle datalink loss via a
 parameter
Merge branch 'master' of github.com:PX4/Firmware into obcfailsafe
WIP, datalinkloss: implementing basic behavior
Merge remote-tracking branch 'upstream/master' into obcfailsafe
Merge remote-tracking branch 'upstream/obcfailsafe' into obcfailsafe
data link loss timeout as param
introduce data link lost counter
abort comm loss mode if counter above param and return home directly
Merge remote-tracking branch 'upstream/master' into obcfailsafe
add method to block fallback to mission

failsafe navigation modes can use a flag in mission_result to tell the
commander to not switch back to mission
datalink loss: fix type error
Merge remote-tracking branch 'upstream/master' into obcfailsafe
navigator: fix comment
dl loss: correct timeout, add hysteresis also for regain
navigator: add skeleton of FW engine failure mode
dl loss: don't set unnecessary value
enginefailure: set mission item
add landengfail nav state
engine_failure flag

Added engine_failure flag to behicle status, alsoset_nav_state in the
state machine helper takes this flag into account