The team is moving utility/library scripts into 'lib' folder.
image_common.sh should follow this policy.
Also refined the parameter check of mk_memento_images.sh
BUG=chromium-os:5208
TEST=./make_factory_package.sh --factory PATH_TO_FACTORY --release PATH_TO_RELEASE;
# factory bundle created successfully
./mk_memento_images.sh PATH_TO_PART2 PATH_TO_PART3
# update.gz created successfully
./mk_memento_images.sh PATH_TO_IMGE 2 3
# update.gz created successfully
Change-Id: I3afecf05da2832986723f28b595045d0540ea9e9
Review URL: http://codereview.chromium.org/4825004