cmake: kconfig: Add APP_DIR to kconfig environment #3022
                
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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
manifest-pr-skip