Config changes for MissionBot

This is a summary and cleanup item for the work @JoeMarino did the last couple weeks, first on the solidoodle with the geeetech printrboard and then on the new CoreXY MissionBot.

- We're leaving [`missionst/solidoodle-printrboard`](https://github.com/MissionSt/Marlin/tree/missionst/solidoodle-printrboard) branch as-is for now, since the Solidoodles were used for prototyping Auto Bed Leveling but we're not using them going forward
- We're also going to leave [`missionst/corexy`](https://github.com/MissionSt/Marlin/tree/missionst/corexy) as is, since it was really experimental and accumulated a lot of cruft
- On Tuesday we had some confusion regarding the configuration changes required to get CoreXY motion working in "landscape" layout on the new MissionBot, but that's been resolved now.
- This pull request is an effort to sort through the changes from the [`missionst/corexy`](https://github.com/MissionSt/Marlin/compare/missionst%2Fmaster...missionst%2Fcorexy) branch and pick out the ones we want going forward
- My initial pass yielded these first 3 commits.  @JoeMarino I'd appreciate it if you could point out anything I've missed in the comments.  It is probably best to look at the diff shown on this page side-by-side with https://github.com/MissionSt/Marlin/compare/missionst%2Fmaster...missionst%2Fcorexy