Small cleanups on the ostree recipes

• Clarify the actual reason machine-id can't be truncated earlier
• Drop the "00000000.0" timestamp defaults
• sdk: Explicitly pass devroot URL
• ostree-commit: Export the list of files that will get committed
• ostree: Subsume ostree-prepare.sh into the recipe itself
• ostree-commit: Provide descriptions for steps that lacked one
• ostree-commit: Commit by default to the uefi board/branch