efi_loader: Increase default variable store size to 64KiB

efi_loader: Increase default variable store size to 64KiB

Debian's arm64 UEFI Secure Boot shim makes the EFI variable store run
out of space while mirroring its MOK database to variables. This can be
observed in QEMU like so:

  $ tools/buildman/buildman -o build/qemu_arm64 --boards=qemu_arm64 -w
  $ cd build/qemu_arm64
  $ curl -L -o debian.iso \
      https://cdimage.debian.org/debian-cd/current/arm64/iso-cd/debian-12.0.0-arm64-netinst.iso
  $ qemu-system-aarch64 \
      -nographic -bios u-boot.bin \
      -machine virt -cpu cortex-a53 -m 1G -smp 2 \
      -drive if=virtio,file=debian.iso,index=0,format=raw,readonly=on,media=cdrom
  [...]
  => # interrupt autoboot
  => env set -e -bs -nv -rt -guid 605dab50-e046-4300-abb6-3dd810dd8b23 SHIM_VERBOSE 1
  => boot
  [...]
  mok.c:296:mirror_one_esl() SetVariable("MokListXRT43", ... varsz=0x4C) = Out of Resources
  mok.c:452:mirror_mok_db() esd:0x7DB92D20 adj:0x30
  Failed to set MokListXRT: Out of Resources
  mok.c:767:mirror_one_mok_variable() mirror_mok_db("MokListXRT",  datasz=17328) returned Out of Resources
  mok.c:812:mirror_one_mok_variable() returning Out of Resources
  Could not create MokListXRT: Out of Resources
  [...]
  Welcome to GRUB!

This would normally be fine as shim would continue to run grubaa64.efi,
but shim's error handling code for this case has a bug [1] that causes a
synchronous abort on at least chromebook_kevin (but apparently not on
QEMU arm64).

Double the default variable store size so the variables fit. There is a
note about this value matching PcdFlashNvStorageVariableSize when
EFI_MM_COMM_TEE is enabled, so keep the old default in that case.

[1] https://github.com/rhboot/shim/pull/577

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>