Strip xattrs from squashfs to fix boot failure

Kernel squashfs driver fails with "unable to read xattr id index table"
when the squashfs contains POSIX ACL xattrs (system.posix_acl_*) written
by mksquashfs as unrecognised entries. This caused every built ISO to
drop to an initramfs shell at boot.

Add -no-xattrs to mksquashfs options so xattrs are stripped at build
time. xattrs are not needed in a live read-only rootfs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/auto/config

@@ -42,4 +42,5 @@ lb config noauto \
     --debootstrap-options "--include=ca-certificates" \
     --apt-recommends false \
     --chroot-squashfs-compression-type zstd \
+    --chroot-squashfs-options "-no-xattrs" \
     "${@}"

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" \
@@ -986,7 +981,6 @@ split_live_squashfs_layers() {
     tmp_root="$(mktemp -d)"
     tmp_usr="$(mktemp -d)"
     tmp_fw="$(mktemp -d)"
-    trap 'rm -rf "$tmp_root" "$tmp_usr" "$tmp_fw"' RETURN
 
     echo "=== splitting live squashfs into smaller layers ==="
     unsquashfs -d "$tmp_root/root" "$base_sq" >/dev/null
@@ -1013,7 +1007,6 @@ split_live_squashfs_layers() {
     echo "=== live squashfs layers ==="
     find "$live_dir" -maxdepth 1 -type f -name '*.squashfs' -exec du -sh {} \; | sort
     rm -rf "$tmp_root" "$tmp_usr" "$tmp_fw"
-    trap - RETURN
 }
 
 recover_iso_memtest() {
@@ -1687,7 +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"