Introduce #defines to make the axis homing against max-endstops able to ...

...stay at the endstop instead of returning to min again.

Mendel 90 for example has Z-endstop at max, defining Z_HOME_MAX here has the effect that the Z-axis stays at the endstop when homing, else the axis would home to Z-max and then lower the print head to zero again, leading to 2 complete length runs of the Z-axis which is slow