Add SBAT metadata to grub-mkstandalone created EFI binaries

grub-mkstandalone supports an --sbat option to embed SBAT metadata.

Even though archiso created ISOs do not support Secure Boot, having SBAT would allow users to more easily repack the files in the ISO to add a signed shim.

The grub package ships with SBAT data in /usr/share/grub/sbat.csv.