This will allow dev who didn't build an image to still use cros_make_image_bootable
to update their image -- including mod_image_for_test / image_to_vm.
Change-Id: I1ba4292223b28b21d5416745e7744641100fa4ad
BUG=7370
TEST=Ran image_to_vm with other image and image I built myself. Also ran
cros_make_image_bootable directly on a the same two images.
Review URL: http://codereview.chromium.org/3569015