Add verity_signer and verity_key flags to add_img_to_target_files.

Add verity_signer and verity_key flags to add_img_to_target_files.

The signing process requires the boot and recovery images be signed
by CROS-Signer, which is done after a signed_target_files.zip is
available.  Once the boot/recovery images are signed, we need
to insert it back into signed_target_files.zip and regenerate the
system.map and verity tree.  The script (add_img_to_target_files) does
this operation and needs to be aware of verity specific flags.

Bug: 23831896
Change-Id: Id24e878161052c768b74da1231b97eb0574b4c8b