Allow disk image target for copy machine.

lib/cros_image_common.sh

Index: lib/cros_image_common.sh
diff --git a/lib/cros_image_common.sh b/lib/cros_image_common.sh
index 86bba755047e612d69916d14876d6df1a6bb0f0e..abfdbc57c9bf7e50f70e2b042220b022c58eda67 100644
--- a/lib/cros_image_common.sh
+++ b/lib/cros_image_common.sh
@@ -168,3 +168,23 @@ image_umount_partition() {
 
   umount -d "$mount_point"
 }
+
+# Copy a partition from one image to another.
+image_partition_copy() {
+  local src="$1"
+  local srcpart="$2"
+  local dst="$3"
+  local dstpart="$4"
+
+  local srcoffset=$(image_part_offset "${src}" "${srcpart}")
+  local dstoffset=$(image_part_offset "${dst}" "${dstpart}")
+  local length=$(image_part_size "${src}" "${srcpart}")
+  local dstlength=$(image_part_size "${dst}" "${dstpart}")
+
+  if [ "${length}" -gt  "${dstlength}" ]; then
+    exit 1
+  fi
+
+  image_dump_partition "${src}" "${srcpart}" |
+      dd of="${dst}" bs=512 seek="${dstoffset}" conv=notrunc
+}