sysbuild: Do not exclude images from domains.yaml

The recent support for BUILD_ONLY images was implemented by excluding
them from `domains.yaml`, in order to crudely prevent them from being
picked up by `west flash`. Arguably, this is incorrect or unexpected,
because the sysbuild documentation defines a "domain" as:

   Every Zephyr CMake build system managed by sysbuild.

Another consequence is that, given a build-only `<image>`, this makes it
impossible to pass `--domain <image>` to `west flash`, `west debug`, and
ironically `west build`.

To fix that, `domains.yaml` should again represent all domains, and the
build-only ones should be indicated in another way. Enter `flash_order`:
a new top-level key in the domains YAML schema. It contains the default
sequence of images used by `west flash`, where the build-only images are
excluded, and the order is influenced by `sysbuild_add_dependencies()`.

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