Add ostree-boot package (#824650)
These changes enables the ostree-boot package. See here for binaries:
https://code.launchpad.net/~fkrull/+archive/ubuntu/ostree-boot
(Yes, it's technically built for Ubuntu, but Launchpad was what I had. It shouldn't make a difference.)
I ended up splitting the package further into ostree-boot and ostree-grub2; the former contains the dracut and systemd boot stuff, the latter contains only the GRUB integration. This means you can install ostree-grub2 on a regular Debian system and have the grub.cfg generation work, without also getting the boot files.
I also weakened the Depends on dracut to a Recommends. That's mostly because the initramfs generators conflict each other, but the systemd bits in ostree-boot might still be useful without dracut.