changes in vehicle with MissionPlanner low battery warning

[tridge]

MissionPlanner has a feature to warn at a voltage level, using the "MP Alert on Low Battery" tick box. If the user changes between a 3S and a 6S vehicle then this warning will persist and be incorrect for the new aircraft. An alternative would be to use the UI to set BATT_LOW_VOLT which is stored on the vehicle, so changes with the vehicle. Alternatively it could be a percentage above the BATT_LOW_VOLT, so the user starts getting a warning at (for example) 10% above the low voltage.