releasetools: Pack updatable images into IMAGES/ for A/B update.

Vendor-specific images (radio, bootloader and etc) used to stay in
RADIO/ in target_files zip. A/B updater expects them to be available
under IMAGES/. Make a copy for such images when calling
add_img_to_target_files.py.

We cannot move them directly from RADIO/ to IMAGES/, because we will
delete everything under IMAGES/ when signing for release.

Bug: 25674625
Change-Id: I977c879d0ff7e45c83b3f5c63844a5b135caa383