integration with rpm-ostree/split bootloader rpms

As noted in the README.md today rpm -q shim-x64 is a lie. Ultimately what we want to do I think is:

• move the bootloader packages out of the manifest into a separate manifest-bootloader.yaml
• add a separate rpm-ostree compose tree run to coreos-assembler using that manifest, writing to a separate ref
• teach cosa's create_disk.sh to use that (we only need that ref if doing a disk image build note!), including passing data from that ref to /usr/libexec/bootupd generate-update-metadata

This would mean that rpm -q shim-x64 would say it's not installed, which is correct - from the perspective of rpm and rpm-ostree. It's not tracked by that system, it's tracked by bootupd. Instead the single source of truth for versioning would then be bootupctl status.