AP_Mission: don't allow waypoint 0 (home) to be changed

[IamPete1]

This highlights what is a tricky problem to fix. The main problem is that this will break most scripts which try and upload missions.

The problem is that mission allows you to set way-point 0, however when you try and read way point 0 you just get home.

ardupilot/libraries/AP_Mission/AP_Mission.cpp

Lines 817 to 822 in 6efe210

	if (index == 0) {
	cmd = {};
	cmd.id = MAV_CMD_NAV_WAYPOINT;
	cmd.content.location = AP::ahrs().get_home();
	return true;
	}

This is the key this issue. You can write whatever you like to waypoint 0, but there is no way to read it back or for it to be used in anyway. (The only way I can think is that if you have a eeprom copy you could read it out manualy)

This makes the change that your no longer allowed to write to waypoint 0, this give users a clue that there throwing away there first point, it will never be used.

Maybe the more complete fix is to stop storing home completely, this would require doing param conversion on the total mission item count.

A nice example of something that does not work as expected on master is the copter save way point switch (you have to make sure your mission is fully cleared and MIS_TOTAL is 0). In this case it carefully checks for there being 0 waypoints and tries to add a takeoff.

ardupilot/ArduCopter/RC_Channel_Copter.cpp

Lines 238 to 250 in 6efe210

	// if the mission is empty save a takeoff command
	if (copter.mode_auto.mission.num_commands() == 0) {
	// set our location ID to 16, MAV_CMD_NAV_WAYPOINT
	cmd.id = MAV_CMD_NAV_TAKEOFF;
	cmd.content.location.alt = MAX(copter.current_loc.alt,100);
	// use the current altitude for the target alt for takeoff.
	// only altitude will matter to the AP mission script for takeoff.
	if (copter.mode_auto.mission.add_cmd(cmd)) {
	// log event
	LOGGER_WRITE_EVENT(LogEvent::SAVEWP_ADD_WP);
	}
	}

However, that takeoff is written to index 0 so it is never used. If you read the mission back again you will find only a single mission item and no take off: