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

scripts/image_signing/common.sh

Index: scripts/image_signing/common.sh
diff --git a/scripts/image_signing/common.sh b/scripts/image_signing/common.sh
index 37834dc541493dfedb067e514a46af9208a1ee8b..c4824c61cc39b35367cad9b84055d1bf1dac1403 100755
--- a/scripts/image_signing/common.sh
+++ b/scripts/image_signing/common.sh
@@ -27,6 +27,16 @@ partsize() {
   sudo $GPT show -s -i $2 $1
 }
 
+# Mount a partition read-only from an image into a local directory
+# Args: IMAGE PARTNUM MOUNTDIRECTORY
+mount_image_partition_ro() {
+  local image=$1
+  local partnum=$2
+  local mount_dir=$3
+  local offset=$(partoffset "$image" "$partnum")
+  sudo mount -o loop,ro,offset=$((offset * 512)) "$image" "$mount_dir"
+}
+
 # Mount a partition from an image into a local directory
 # Args: IMAGE PARTNUM MOUNTDIRECTORY
 mount_image_partition() {