sysbuild: Remove IMAGES variable

This variable was originally provided for two indended purposes:

  * Let users manually add a new image in a desired order in the list.
  * Let users set build-only images, which are excluded from the list.

Given the recent additions of the `sysbuild_add_dependencies()` function
and the `BUILD_ONLY` parameter, `IMAGES` should be considered obsolete.

Furthermore, the list of images added to sysbuild should be updated
automatically when calling `ExternalZephyrProject_Add()`. This is now
possible by using a GLOBAL property internal to sysbuild.

With that, the `IMAGES` variable can be removed. Its existing usage for
image ordering is replaced with `sysbuild_add_dependencies()` treewide.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>