cros_generate_upload_payload --patch_kernel should mount from the source

cros_generate_update_payload

 #!/bin/bash
 
 # Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Script to generate a Chromium OS update for use by the update engine.
 # If a source .bin is specified, the update is assumed to be a delta update.
 
 # Load common constants.  This should be the first executable line.
@@ skipping 69 matching lines @@
 patch_kernel() {
   local IMAGE="$1"
   local KERN_FILE="$2"
 
   STATE_LOOP_DEV=$(sudo losetup -f)
   [ -n "$STATE_LOOP_DEV" ] || die "no free loop device"
   local offset=$(partoffset "${IMAGE}" 1)
   offset=$(($offset * 512))
   sudo losetup -o "$offset" "$STATE_LOOP_DEV" "$IMAGE"
   STATE_MNT=$(mktemp -d /tmp/state.XXXXXX)
-  sudo mount "$STATE_LOOP_DEV" "$STATE_MNT"
+  sudo mount --read-only "$STATE_LOOP_DEV" "$STATE_MNT"
   dd if="$STATE_MNT"/vmlinuz_hd.vblock of="$KERN_FILE" conv=notrunc
   sudo umount "$STATE_MNT"
   STATE_MNT=""
   sudo losetup -d "$STATE_LOOP_DEV"
   STATE_LOOP_DEV=""
 }
 
 extract_kern_root() {
   local bin_file="$1"
   local kern_out="$2"
@@ skipping 137 matching lines @@
 
   GENERATOR="$(dirname "$0")/mk_memento_images.sh"
 
   CROS_GENERATE_UPDATE_PAYLOAD_CALLED=1 "$GENERATOR" "$DST_KERNEL" "$DST_ROOT"
   mv "$(dirname "$DST_KERNEL")/update.gz" "$FLAGS_output"
 
   trap - INT TERM EXIT
   cleanup noexit
   echo "Done generating full update."
 fi