Add an option for delta to home to the top

[thinkyhead]

Some users will prefer delta homing to stay at the top instead of lowering below the clipped area.
This PR adds the option DELTA_HOME_TO_SAFE_ZONE to restore that behavior.

[Barrocas]

Thanks :)
I'm building a new Delta, and I don't use G29 (yet).
I'm also using spectra fishing line instead of belts, along with custom made pulleys and idlers, and a 3 point adjustable bed, so having the homing stay at the top is nicer to help in getting the Zmax height, adjust the bed, and calculate correct steps/mm.