[nrf fromlist] 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.

Upstream PR #: 92674

Signed-off-by: Joakim Andersson <joerchan@gmail.com>

Author: joerchan

cmake/modules/kconfig.cmake

@@ -142,6 +142,7 @@ set(COMMON_KCONFIG_ENV_SETTINGS
   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}