[nrf fromtree] cmake: kconfig: Add APP_DIR to kconfig environment

Add APP_DIR as a kconfig environment variable.

This is useful in cases where you want to set a config path relative to
the directory of the application.

An example of this is how many sysbuild.cmake files sets the source
directory like this:

ExternalZephyrProject_Add(
  APPLICATION remote
  SOURCE_DIR ${APP_DIR}/remote
  BOARD ${SB_CONFIG_REMOTE_BOARD}
)

The same however cannot be done in Kconfig.sysbuild:

config NETCORE_IMAGE_PATH
    default "${APP_DIR}/<image_path>" if NETCORE_ABC

Instead they must use ZEPHYR_MY_MODULE_MODULE_DIR, however not all
applications are part of a zephyr module.

Signed-off-by: Joakim Andersson <joerchan@gmail.com>
(cherry picked from commit 12fdde6694e7a1be065d65c335aeadf7129e34bf)

Author: joerchan

cmake/modules/kconfig.cmake

@@ -135,13 +135,17 @@ else()
   set(_local_TOOLCHAIN_HAS_PICOLIBC n)
 endif()
 
+# APP_DIR: Path to the main image (sysbuild) or synonym for APPLICATION_SOURCE_DIR (non-sysbuild)
+zephyr_get(APP_DIR VAR APP_DIR APPLICATION_SOURCE_DIR)
+
 set(COMMON_KCONFIG_ENV_SETTINGS
   PYTHON_EXECUTABLE=${PYTHON_EXECUTABLE}
   srctree=${ZEPHYR_BASE}
   KERNELVERSION=${KERNELVERSION}
   APPVERSION=${APP_VERSION_STRING}
   APP_VERSION_EXTENDED_STRING=${APP_VERSION_EXTENDED_STRING}
   APP_VERSION_TWEAK_STRING=${APP_VERSION_TWEAK_STRING}
+  APP_DIR=${APP_DIR}
   CONFIG_=${KCONFIG_NAMESPACE}_
   KCONFIG_CONFIG=${DOTCONFIG}
   KCONFIG_BOARD_DIR=${KCONFIG_BOARD_DIR}