Making G28 override optional (configs will need updating)

[jschuh]

I recently added a LAZY_HOME macro, and am planning on moving the G28 override to the optional directory. This means that anyone with homing_override section will need to include that optional file or reverse the instructions from the readme.

I'm planning on doing this because I've just never been happy with the hacky way that G28 has to be overridden. I'm still thinking about how to do this in a way that's minimally disruptive. And I will of course add console messages pointing to detailed instructions on how people can fix their configs.