Implement raise/lower for default keymap.

* Properly allow includes for keymaps to allow defining
  custom keycodes, and then adding them to the
  keymap.overlay file.
* Fix keymap.c loading after build refactor.
* Fix for deactivating a layer working properly.
* Fix default keymap to put transparent in proper spots to
  make raise/lower keycodes work as expected.
* Add custom raise/lower keycode handling to default
  keymap.

CMakeLists.txt

@@ -9,9 +9,9 @@ include(cmake/keymap.cmake)
 find_package(Zephyr)
 project(zmk)
 
-if(EXISTS ${KEYMAP_DIR}/${KEYMAP}/keymap.c)
+if(EXISTS ${KEYMAP_DIR}/keymap.c)
 	message(STATUS "ADDING THE KEYMAP SOURCE")
-	target_sources(app PRIVATE ${KEYMAP_DIR}/${KEYMAP}/keymap.c)
+	target_sources(app PRIVATE ${KEYMAP_DIR}/keymap.c)
 endif()