Revert squashfs layer split — live-boot cannot mount partial rootfs

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>

iso/builder/build.sh

@@ -896,11 +896,6 @@ needs_full_build() {
     [ -f "${FULL_BUILD_MARKER}" ]                                        || return 0
     [ -f "${BUILD_WORK_DIR}/binary/live/filesystem.squashfs" ]           || return 0
     [ -f "${BUILD_WORK_DIR}/live-image-amd64.hybrid.iso" ]               || return 0
-    _extra_sq=$(find "${BUILD_WORK_DIR}/binary/live" -maxdepth 1 -type f -name '*.squashfs' ! -name 'filesystem.squashfs' 2>/dev/null | head -1)
-    if [ -n "$_extra_sq" ]; then
-        echo "=== full build required: multi-squashfs live image present ==="
-        return 0
-    fi
 
     _heavy=$(find \
         "${BUILDER_DIR}/VERSIONS" \
@@ -1685,7 +1680,6 @@ run_step_sh "live-build clean" "80-lb-clean" "lb clean --all 2>&1 | tail -3"
 run_step_sh "live-build config" "81-lb-config" "lb config 2>&1 | tail -5"
 dump_memtest_debug "pre-build" "${LB_DIR}"
 run_step_sh "live-build build" "90-lb-build" "lb build 2>&1"
-split_live_squashfs_layers "${LB_DIR}"
 echo "=== enforcing canonical bootloader assets ==="
 enforce_live_build_bootloader_assets "${LB_DIR}"
 reset_live_build_stage "${LB_DIR}" "binary_checksums"