[Merge] ~xnox/livecd-rootfs:fix-grub-images into livecd-rootfs:ubuntu/master

[Dimitri John Ledkov]

Tested that amd64 and arm64 images work correctly with this change.
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https://code.launchpad.net/~xnox/livecd-rootfs/+git/livecd-rootfs/+merge/401779
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~xnox/livecd-rootfs:fix-grub-images into livecd-rootfs:ubuntu/master.