I made a blogpost detailing the steps to debug PCI Option ROM with Coreboot+SeaBIOS and a GDB-server-compatible debugger:
http://bioshacking.blogspot.com/search/label/PCI%20Option%20ROM
Hopefully would help those in need because it takes quite a while to get it right. Thanks to Kevin O'Connor for the helps :-)
Regards,
Darmawan