Use descriptive constants instead of numeric literals
Use descriptive constants, more
Prevent G29 without first homing in X and Y

If position is unknown, then G29 can be dangerous.
update comment
Merge pull request #794 from MissionSt/g29_requires_home

Prevent G29 without first homing in X and Y
Merge pull request #793 from MissionSt/use_axis_enums

Use constants instead of numeric literals
Merge remote-tracking branch 'upstream/Marlin_v1' into Marlin_v1