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/CMakeLists.txt

@@ -2,14 +2,7 @@ cmake_minimum_required(VERSION 3.13.1)
 
 set(CONFIG_APPLICATION_DEFINED_SYSCALL true)
 
-# Add our custom Zephyr module for drivers w/ syscalls, etc.
-list(APPEND DTS_ROOT ${CMAKE_SOURCE_DIR}/drivers/zephyr)
-
-set(ZephyrBuildConfiguration_ROOT ${CMAKE_SOURCE_DIR}/cmake)
-
-list(APPEND ZEPHYR_EXTRA_MODULES
-  ${CMAKE_CURRENT_SOURCE_DIR}/module
-)
+set(ZEPHYR_EXTRA_MODULES "${ZMK_EXTRA_MODULES};${CMAKE_CURRENT_SOURCE_DIR}/module;${CMAKE_CURRENT_SOURCE_DIR}/keymap-module")
 
 # Find Zephyr. This also loads Zephyr's build system.
 find_package(Zephyr REQUIRED HINTS ../zephyr)

app/cmake/ZephyrBuildConfig.cmake → app/keymap-module/modules/modules.cmake

@@ -5,8 +5,6 @@
 #    * single overlay,
 #    * or per board/shield.
 
-cmake_minimum_required(VERSION 3.15)
-
 list(APPEND BOARD_ROOT ${APPLICATION_SOURCE_DIR})
 list(APPEND DTS_ROOT ${APPLICATION_SOURCE_DIR})
 

app/keymap-module/zephyr/module.yml

@@ -0,0 +1,3 @@
+build:
+  settings:
+    module_ext_root: "."