* Handle board keymap location for boards with Zephyr
board revisions included.
* Includes bare non-revision file and a revision specific keymap
in case newer revision changes the layout/key positions.
* Among other issues, this message is often misinterpreted by users
building out-of-tree shields -- leading them to think the shield
"not being found" is the cause of a build failure.
* Shift to using an extra Zephyr module to do keymap location
work after all board roots are resolved. This avoids duplicate work
and allows us to load custom boards from Zephyr modules as well as
user config setups.
