[Merge] ~ikapelyukhin/livecd-rootfs/+git/livecd-rootfs:master-unset-initrdless-boot-fallback into livecd-rootfs:ubuntu/master

[Ivan Kapelyukhin]

Ivan Kapelyukhin has proposed merging ~ikapelyukhin/livecd-rootfs/+git/livecd-rootfs:master-unset-initrdless-boot-fallback into livecd-rootfs:ubuntu/master.

Commit message:
fix: unset initrdless_boot_fallback_triggered instead of setting it to 0

This fixes GCE shielded VM instances integrity monitoring failures on
focal and later. Our images are built with an empty /boot/grub/grubenv
file, however after the first boot `initrdless_boot_fallback_triggered`
is set to 0. This change in `grubenv` results in integrity monitoring
`lateBootReportEvent` error.

It seems that the only thing that's checking for this `grubenv` variable
is `grub-common.service`, and it is looking specifically for a `1`
value:

  if grub-editenv /boot/grub/grubenv list | grep -q
  initrdless_boot_fallback_triggered=1; then echo "grub:
  GRUB_FORCE_PARTUUID set, initrdless boot paniced, fallback triggered.";
  fi

Unsetting this variable instead of setting it to 0 would prevent issues
with integrity monitoring.


Requested reviews:
  Ubuntu Core Development Team (ubuntu-core-dev)
Related bugs:
  Bug #1960564 in livecd-rootfs (Ubuntu): "GCE shielded VM integrity monitoring reports errors"
  https://bugs.launchpad.net/ubuntu/+source/livecd-rootfs/+bug/1960564

For more details, see:
https://code.launchpad.net/~ikapelyukhin/livecd-rootfs/+git/livecd-rootfs/+merge/415413
-- 
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~ikapelyukhin/livecd-rootfs/+git/livecd-rootfs:master-unset-initrdless-boot-fallback into livecd-rootfs:ubuntu/master.
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