Make the flight node go to stand-by state instead of exiting when LAND mode is triggered

[johankytt]

The logic should be something like that:

1. Land mode observed
2. Stop flight loop
3. Enter some kind of stand-by state an wait for motors to disarm
4. Wait for motors to be armed
5. When armed status is detected, start the flight loop again

Care must be taken that once LAND mode is triggered, the flight loop does not re-activate until the drone has landed and the motors have been re-armed.