Use local apt cache instead of local_repo when mastering an image.

src/scripts/build_image.sh

 #!/bin/bash
 
 # Copyright (c) 2009 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 build a bootable keyfob-based chromeos system image.
 # It uses debootstrap (see https://wiki.ubuntu.com/DebootstrapChroot) to
 # create a base file system. It then cusotmizes the file system and adds
 # Ubuntu and chromeos specific packages. Finally, it creates a bootable USB
@@ skipping 15 matching lines @@
 DEFINE_integer build_attempt 1                                \
   "The build attempt for this image build."
 DEFINE_string output_root "${DEFAULT_BUILD_ROOT}/images"      \
   "Directory in which to place image result directories (named by version)"
 DEFINE_string build_root "$DEFAULT_BUILD_ROOT"                \
   "Root of build output"
 DEFINE_boolean replace $FLAGS_FALSE "Overwrite existing output, if any."
 DEFINE_boolean increment $FLAGS_FALSE \
   "Picks the latest build and increments the minor version by one."
 
-DEFINE_string mirror "$DEFAULT_IMG_MIRROR" "Repository mirror to use."
+DEFINE_string mirror "$DEFAULT_EXT_MIRROR" "Repository mirror to use."
 DEFINE_string suite "$DEFAULT_IMG_SUITE" "Repository suite to base image on."
 DEFINE_string pkglist "$DEFAULT_PKGLIST" \
   "Name of file listing packages to install from repository."
 
 # Parse command line
 FLAGS "$@" || exit 1
 eval set -- "${FLAGS_ARGV}"
 
 # Die on any errors.
 set -e
@@ skipping 122 matching lines @@
 EOF
 
 OUTSIDE_OUTPUT_DIR="${EXTERNAL_TRUNK_PATH}/src/build/images/${IMAGE_SUBDIR}"
 echo "Done.  Image created in ${OUTPUT_DIR}"
 echo "To copy to USB keyfob, outside the chroot, do something like:"
 echo "  ./image_to_usb.sh --from=${OUTSIDE_OUTPUT_DIR} --to=/dev/sdb"
 echo "To convert to VMWare image, outside the chroot, do something like:"
 echo "  ./image_to_vmware.sh --from=${OUTSIDE_OUTPUT_DIR}"
 
 trap - EXIT