Fix validate_target_files for target files modified by

add_img_to_target_files.py

See bug description:
"When resigning images, the validate_target_files.py can be used to
verify the images using avbtool. The script will use the vbmeta.img to
achieve this, and all relevant images need to be in the IMAGES folder.
However, due to changes on add_img_to_target_files.py and specifically
the commit 5277d101, some images (e.g. acpio.img and tos.img) are no
longer copied from RADIO to the IMAGES folder. This causes an error on
validate_target_files.py indicating that it cannot find image such as
IMAGES/acpio.img."
This CL fixes this by symlink images under RADIO directory to IMAGES
directory before invoking avbtool.

Bug: 159299583
Test: python3 -m unittest test_validate_target_files
Change-Id: I5769ee2ab5230d2a3a7cef10706dcc5788e654f8