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 branch 'Marlin_v1' of https://github.com/ErikZalm/Marlin into typofixes_upstream