mission feasibility checker: add check for waypoint altitude relative to home position altitude
mission feasibility checker: remove audio warning for waypoint below home altitude
mission feasibility checker: add missing default return in checkHomePositionAltitude
Merge pull request #694 from thomasgubler/beta_homealtitudewarning

mission feasibility checker: add warning if waypoint is below home altitude
Merge pull request #1019 from PX4/beta

mission feasibility checker: add warning if waypoint is below home altitude