Use Endless version of OStree, plus some small fixups.

[ghost]

Use https://github.com/endlessm/ostree, which is needed for #11.

I've tested image building and upgrades from eos6 with this change and found no issues.

Two tiny changes included as well:

• Remove an obsolete element
• Update TEST_MATRIX now that Secure Boot works

[starnight]

EOS PAYG image uses sd-boot + custom initramfs & kernel efi blobs + OSTree.

Current EOS ostree has downstream modification:

• sd-boot + OSTree: (Upstream has sysroot: Support for directories instead of symbolic links in boot part discussing about it)
  • deploy: Create fake symlinks when on FAT filesystems
  • deploy: Don't try to create symlinks on FAT /boot
• custom initramfs & kernel efi blobs + OSTree:
  • deploy: Handle efi blobs

Comparing to freedesktop's OSTree, this PR will downgrade it. But, it is a quick way to make EOS PAYG image work. Well, still need other components, too. Such as, sd-boot.

[ghost]

Right, I hadn't spotted the systemd patches, but handling that separately in #161