corrected homing for using max endstops

BACK_MOVES didn't work when using MAX-ENDSTOPS because during the first approach on an endstop the acutal position is set to 0.
Update Commands.cpp
Merge pull request #1 from sivar2311/homing_testing

Fixed: Homing with MAX-Endstops
Merge pull request #2 from sivar2311/master

0.83 homing for max-endstops fixed