Consider search_path to locate avb_*_key_path files

avb_*_key_path are usually relative file paths. If these files cannot be
located from the current directory, try prepending OPTIONS.search_path.

Bug: 139836256
Test: python -m unittest test_common
Test: python -m unittest test_add_img_to_target_files
Change-Id: I020b257f458a1f5762691d0499feaf4027e37126