refactor: Hook into CMake loading in a better spot.

* 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.

app/keymap-module/zephyr/module.yml

@@ -0,0 +1,5 @@
+# This ensures our modules/modules.cmake file is loaded *after* all the other modules,
+# To set up the various keymap DTS and overridden .conf files are located and chosen.
+build:
+  settings:
+    module_ext_root: "."