Skip copying existing vendor images

add_img_to_target_files.py has an option of "-a" to add missing
images only. Under this option, the script should skip copying
the radio images for A/B devices when given image exists already
under "IMAGES/".

Test: Run the command on an A/B device, the existing radio images under "IMAGES/" don't get overwritten; and missing images are added correctly.
Bug: 29608905
Change-Id: Ie034b85a5d777d53e367f99470cea4d19cb1aaaf