am 6f1d031b: take bootable images from target_files if present

                     info_dict=None):

  """Return a File object (with name 'name') with the desired bootable

  image.  Look for it in 'unpack_dir'/BOOTABLE_IMAGES under the name

  'prebuilt_name', otherwise construct it from the source files in

  'prebuilt_name', otherwise look for it under 'unpack_dir'/IMAGES,

  otherwise construct it from the source files in

  'unpack_dir'/'tree_subdir'."""

  prebuilt_path = os.path.join(unpack_dir, "BOOTABLE_IMAGES", prebuilt_name)

  if os.path.exists(prebuilt_path):

    print "using prebuilt %s..." % (prebuilt_name,)

    print "using prebuilt %s from BOOTABLE_IMAGES..." % (prebuilt_name,)

    return File.FromLocalFile(name, prebuilt_path)

  else:

  prebuilt_path = os.path.join(unpack_dir, "IMAGES", prebuilt_name)

  if os.path.exists(prebuilt_path):

    print "using prebuilt %s from IMAGES..." % (prebuilt_name,)

    return File.FromLocalFile(name, prebuilt_path)

  print "building image from target_files %s..." % (tree_subdir,)

  fs_config = "META/" + tree_subdir.lower() + "_filesystem_config.txt"

  data = BuildBootableImage(os.path.join(unpack_dir, tree_subdir),