Pre-bind bootloader to bootfs at installation time

[jlebon]

Feature Request

Desired Feature

Now that we support binding the bootloader to the bootfs, we can do some of that work upfront at installation time. This will ensure that the correct bootfs is booted by the bootloader on first boot, and that the correct bootfs is mounted by the initramfs.

Other Information

• This will require restamping the bootfs UUID. The FCOS first boot code should already be able to handle non-factory default UUIDs.
• This work does not obviate coreos-boot-edit: abort if system has several boot partitions fedora-coreos-config#1269; we still want to error out if multiple boot filesystems are found. This RFE would allow us to more effectively "funnel" that error state to rdcore verify-unique-fs-label rather than failing earlier on an obscure bootloader or bootfs mismatch issue.