Add support for ext4 XBOOTLDR partitions

[ermo]

It turns out systemd-boot might support plugins that would enable us to use ext4 (instead of vfat) XBOOTLDR (/boot) partitions.

It basically involves adding EFI/systemd/drivers/ext2_x64.efi from here.

Also, right now, blsforme is hardcoded to assume that the XBOOTLDR partition is VFAT, which means it kills the ability to boot if the partition is, say, ext4 (don't ask me how I know... 😖 )

The ability to use hardlinks/symlinks on ext4 /boot partitions might open some nice avenues of development down the line.