Fix LAYOUT define generation (#4148)

Fix LAYOUT define generation (#4148)

* Fix LAYOUT define generation

Prior to this, only the full keyboard path was defined.  Eg `KEYBOARD_planck_rev6`.
But the docs mention `KEYBOAD_planck`, which never actually gets defined.

This addresses this, and creates a define for each level of the keyboard folder, so that stuff like `KEYBOARD_planck` actually exists.

* Clean up changes to make it more readable