Scrambled display with newer grubx64.efi bootloader and terminal_output gfxterm

[Alexandros Afentoulis]

Hello and thanks for maintaining grub in Ubuntu.

We've been using grub EFI boot loader in Bionic and Focal official cloud images. After upgrading grub packages from 2.02-2ubuntu8.14 to 2.02-2ubuntu8.17 we are having trouble with console display (provided by QEMU VNC) as it is scrambled and unreadable. See attached screenshot.

We have noticed that the problem manifests itself when terminal_output is set to "gfxterm" but is not present when set to "console". Additionally we verified that in Bionic the problem manifests itself with boot loader provided by 1.93.19+2.02-2ubuntu8.17 (sha1sum: 7be316b0542921956dc3742d70b26e409ca83ef0 /boot/efi/EFI/BOOT/grubx64.efi) while the boot loader provided by 1.93.15+2.02-2ubuntu8.14 (sha1sum: 2895a67157d97b0d3825987639d817a08aad3da8 /boot/efi/EFI/BOOT/grubx64.efi) works just fine.

Problem manifests itself in Focal hosts as well featuring grub-efi-amd64-signed version 1.142.4+2.04-1ubuntu26.2.

Boot loader and related files are installed by grub in /boot/efi/EFI/BOOT/ during the image creation like this:

```
grub-install --boot-directory=/boot --efi-directory=/boot/efi --target=x86_64-efi --removable --uefi-secure-boot --no-nvram
```

said invocation among other things does copy files provided by grub-efi-amd64-signed package:
```
grub-install: info: copying `/usr/lib/grub/x86_64-efi-signed/gcdx64.efi.signed' -> `/boot/efi/EFI/BOOT/grubx64.efi'.
grub-install: info: copying `/usr/lib/shim/shimx64.efi.signed' -> `/boot/efi/EFI/BOOT/BOOTX64.EFI'.
grub-install: info: copying `/usr/lib/shim/mmx64.efi' -> `/boot/efi/EFI/BOOT/mmx64.efi'.
grub-install: info: copying `/usr/lib/shim/BOOTX64.CSV' -> `/boot/efi/EFI/BOOT/BOOTX64.CSV'.
grub-install: info: copying `/boot/grub/x86_64-efi/load.cfg' -> `/boot/efi/EFI/BOOT/grub.cfg'.
```

A naive experimentation, swapping the old boot loader in place of the newer, restores display back to normal.

We will be happy to provide more details if needed.