Added home direction logic

[T3P3]

The direction which an axis homes in is either "-1" for min or "1" for max

The Mendel90 homes max on all axis, hence:

define HOME_DIRECTION {1,1,1,1}

for the original logic use:

define HOME_DIRECTION {-1,-1,-1,-1}

[T3P3]

Drat, github merges commits into a single pull request. The second commit is to allow for the Z probe to be used selectively

[jmgiacalone]

I think this would be better with an M code to toggle use of the Z probe. This would allow for a machine which had a physical microswitch as well as a proximity sensor.

[T3P3]

Can do M31 seems like a good candidate

[T3P3]

Closed due to significant upstream changes