scripts/signrom.py: Allow option ROM checksum script to write the size header.

Modify the signrom.py script so that if the size byte in the header is
0 (ie. not set) then the script will set the size.  If the size byte
is non-zero then we do the same as before, so this doesn't require
changes to any existing ROM sourcecode.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Message-Id: <1463000807-18015-2-git-send-email-rjones@redhat.com>