[rhcos-4.8] buildextend-live: drop shim fallback.efi from ISO

[bgilbert]

UEFI boots from removable media via the arch-specific default EFI application in /EFI/BOOT. When booted that way, shim chains to fallback.efi if it exists, and fallback.efi creates an EFI boot entry. That's not appropriate for removable media boot, since the media will probably never be present again. If a TPM is present, fallback.efi will additionally reboot the machine, and on some machines this leads to boot loops. Instead of all this, we just want shim to chain directly to GRUB.

Drop fallback.efi and its associated CSV from the EFI image. Replace it with a copy of GRUB in the right place for shim to chain to it.

This is a lightweight backport of #2435.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2004449

[miabbott]

Built locally, then build RHCOS 4.8. Verified the ISO boots in UEFI/BIOS. Confirmed that the fallback EFI binary and CSV file were missing.

$ buildah bud --pull-always -t localhost/coreos-assembler:rhcos-4.8 .
$ coreos-assembler fetch && coreos-assembler build && coreos-assembler buildextend-metal && coreos-assembler buildextend-metal4k && coreos-assembler buildextend-live
$ coreos-assembler run -p qemu-iso --qemu-firmware uefi --memory 8192 --devshell-console
$ coreos-assembler run -p qemu-iso --memory 8192 --devshell-console
$ sudo mount -o loop builds/latest/x86_64/rhcos-48.84.202109161903-0-live.x86_64.iso /mnt/iso
$ sudo mount -o loop,ro /mnt/iso/images/efiboot.img /mnt/efi
$ tree /mnt/efi
/mnt/efi
└── EFI
    ├── BOOT
    │   ├── BOOTX64.EFI
    │   └── grubx64.efi
    └── redhat
        ├── fonts
        ├── grub.cfg
        ├── grubx64.efi
        ├── mmx64.efi
        ├── shimx64.efi
        └── shimx64-redhat.efi

4 directories, 7 files