split_live_squashfs_layers moved /usr out of filesystem.squashfs into a
separate 10-usr.squashfs, leaving a rootfs skeleton that live-boot
(1:20230131+deb12u1) cannot mount: the initramfs panics with
"Can not mount /dev/loop0 ... filesystem.squashfs".
live-boot in bookworm expects a single self-contained filesystem.squashfs.
Revert to the standard single-squashfs layout and remove the dead
multi-squashfs guard in needs_full_build().
The split_live_squashfs_layers function is kept for future reference.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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