cmake: modules: configuration_files: Force set app config dir

Forces setting the cached APPLICATION_CONFIG_DIR variable so that
it gets updated with the absolute path instead of reverting back
to the relative path which then causes problems when sourcing
other files, e.g. app.overlay

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/cmake/modules/configuration_files.cmake b/cmake/modules/configuration_files.cmake
index ba62081..76d19e5 100644
--- a/cmake/modules/configuration_files.cmake
+++ b/cmake/modules/configuration_files.cmake
@@ -83,7 +83,7 @@
   zephyr_file(CONF_FILES ${APPLICATION_CONFIG_DIR}/boards KCONF CONF_FILE BUILD ${CONF_FILE_BUILD_TYPE})
 endif()
 
-set(APPLICATION_CONFIG_DIR ${APPLICATION_CONFIG_DIR} CACHE INTERNAL "The application configuration folder")
+set(APPLICATION_CONFIG_DIR ${APPLICATION_CONFIG_DIR} CACHE INTERNAL "The application configuration folder" FORCE)
 set(CACHED_CONF_FILE ${CONF_FILE} CACHE STRING "If desired, you can build the application using\
 the configuration settings specified in an alternate .conf file using this parameter. \
 These settings will override the settings in the application’s .config file or its default .conf file.\