M206 isn't persistent

[RRP-support]

M206 just defines a move, then sets the co-ordinates it has moved from as the current location. The offset is lost as soon as you home the axes again. It would be far more convenient to be able to define the offset once in the config.g and for it to be persistent, rather than having to edit all of the homing macros, so it does it after every homing command. This would mimic use of the command in Marlin, then.