set_lsb_release.sh: Make it mount rootfs r/w only if necessary.

scripts/image_signing/set_lsb_release.sh

 #!/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.
 
 # Customizes a Chrome OS release image by setting /etc/lsb-release values.
 
 # Load common constants and variables.
 . "$(dirname "$0")/common.sh"
@@ skipping 30 matching lines @@
 
 Sets the CHROMEOS_RELEASE_DESCRIPTION key's value to "New description"
 in /etc/lsb-release in chromiumos_image.bin, sorts the keys and dumps
 the updated file to stdout.
 
 EOF
     exit 1
   fi
 
   local rootfs=$(mktemp -d)
-  mount_image_partition "$image" 3 "$rootfs"
+  mount_image_partition_ro "$image" 3 "$rootfs"
   trap "sudo umount -d $rootfs; rm -rf $rootfs" EXIT
   if [ -n "$key" ]; then
+    sudo umount -d "$rootfs"
+    mount_image_partition "$image" 3 "$rootfs"
     set_lsb_release_keyval "$rootfs" "$key" "$value"
     touch "$image"  # Updates the image modification time.
   fi
   cat "$rootfs/etc/lsb-release"
 }
 
 main "$@"