When generating an osbuild manifest for an image type, we take a
customizations struct, which specifies the image-type-independent
customizations to apply. We also take the size argument, which is
specific to the image build and not part of the blueprint.
Introduce a new argument ImageOptions, which for now just wraps the size
argument. These options are specific to the image build/type, and
therefore does not belong with the other customizations.
For now this is a non-functional change, but follow-up commits will
introduce more types of image options.
Signed-off-by: Tom Gundersen <teg@jklm.no>
