- enabled via new param LAND_ABORT_THR default is 0 (disabled)
- Triggered via 95% throttle during landing, a landing abort will take place.
- This copies all takeoff params for right now, we can make this better later if needed
- added mission item command to NAV_LAND which is the abort takeoff altitude. If 0 then use last takeoff if available, else use 30m
- hold heading, just like takeoff, until altitude is reached
- pitch is constrained to takeoff pitch, or else 10deg if not available
- After abort altitude is reached, the normal landing restart happens (DO_LAND_START or decrement mission)
- restart landing by jumping to DO_LAND_START or decrement mission on mode change
If someone mistakenly puts all 0's in their LAND command then
total_distance will be calculated as 0 and cause a divide by 0 error
below thus crashing ArduPilot. Lets avoid that.