mkosi: Reuse main image prepare scripts in subimages

mkosi: Reuse main image prepare scripts in subimages

In the subimages we also want to make sure all dependencies of the
systemd packages are cached so reuse the same prepare scripts from
the main image to do that.

We only want required dependencies in the subimages, not recommended
or suggested dependendencies, so add an environment variable
$SYSTEMD_REQUIRED_DEPS_ONLY which the prepare scripts can check for
and enable it for the subimages.