mod_image_for_factor: never touch firmware for factory test images we don't want any possibility for firmware to be changed in factory setup BUG=none TEST=build_image; image_to_usb --factory Change-Id: Ibef767ea621193afb7fbca5ea96120a506891dcb R=nsanders@chromium.org Review URL: http://codereview.chromium.org/6764010