Add AFTL relevant options to OPTIONS in common.py.

Adds AFTL relevant options in preparation for avbtool changes. The
options are:
aftl_server: A string representing the URL of the transparency log.
aftl_key_path: The path to the public key associated with the
transparency log (in PEM format).
aftl_manufacturer_key_path: The path to the key associated with the
manufacturer for AFTL message signing.
aftl_signing_helper: The signing helper for use with AFTL message
signing.

Change-Id: I73d466f24b171c16dbdaa40c589779bed52be551
Test: Ran add_img_to_target_files with all AFTL options.
Bug: 138779249