Add a script to generate builds signed using the official keys.

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
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+#!/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.
+
+# Determine script directory
+SCRIPT_DIR=$(dirname $0)
+GPT=cgpt
+
+# Read GPT table to find the starting location of a specific partition.
+# Args: DEVICE PARTNUM
+# Returns: offset (in sectors) of partition PARTNUM
+partoffset() {
+  sudo $GPT show -b -i $2 $1
+}
+
+# Read GPT table to find the size of a specific partition.
+# Args: DEVICE PARTNUM
+# Returns: size (in sectors) of partition PARTNUM
+partsize() {
+  sudo $GPT show -s -i $2 $1
+}
+
+# Mount a partition from an image into a local directory
+# Args: IMAGE PARTNUM MOUNTDIRECTORY
+mount_image_partition() {
+  local image=$1
+  local partnum=$2
+  local mount_dir=$3
+  local offset=$(partoffset "$image" "$partnum")
+  sudo mount -o loop,offset=$((offset * 512)) "$image" "$mount_dir"
+}
+
+# Extract a partition to a file
+# Args: IMAGE PARTNUM OUTPUTFILE
+extract_image_partition() {
+  local image=$1
+  local partnum=$2
+  local output_file=$3
+  local offset=$(partoffset "$image" "$partnum")
+  local size=$(partsize "$image" "$partnum")
+  dd if=$image of=$output_file bs=512 skip=$offset count=$size
+}
+