sysbuild: check for existence of APPLICATION_CONFIG_DIR

Check for existence of APPLICATION_CONFIG_DIR in sysbuild.
sysbuild.conf is an optional file, meaning that sysbuild will not fail
if APPLICATION_CONFIG_DIR points to a non-existing folder.

A normal Zephyr build already fails when passing an invalid
APPLICATION_CONFIG_DIR but only because it cannot find a required
prj.conf in the folder.

Checking the existence of APPLICATION_CONFIG_DIR in sysbuild and fail
the build when a non-existing folder is passed to sysbuild ensures
sysbuild behaves similar to a normal Zephyr build regarding a
non-existing folder, while still allow sysbuild.conf itself to be
optional.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

share/sysbuild/cmake/modules/sysbuild_kconfig.cmake

@@ -73,6 +73,13 @@ if(NOT DEFINED SB_CONF_FILE)
  set(SB_CONF_FILE ${CMAKE_CURRENT_BINARY_DIR}/empty.conf)
 endif()
 
+if(NOT EXISTS "${APPLICATION_CONFIG_DIR}")
+ message(FATAL_ERROR "${APPLICATION_CONFIG_DIR} (APPLICATION_CONFIG_DIR) doesn't exist. "
+ "Unable to lookup sysbuild.conf or other related sysbuild configuration files. "
+ "Please ensure APPLICATION_CONFIG_DIR points to a valid location."
+ )
+endif()
+
 # Empty files to make kconfig.py happy.
 file(TOUCH ${CMAKE_CURRENT_BINARY_DIR}/empty.conf)
 set(APPLICATION_SOURCE_DIR ${sysbuild_toplevel_SOURCE_DIR})