I used to use ps2_unpacker, but it was getting really hard to find that tool when I checked the last time. You could use the tool I wrote: - ROMIMG support page -
Originally Posted by gmack101
But do you know what you're really doing? One can't flash the "BIOS" back into a PlayStation 2 console without replacing that entire chip.
And there isn't really much of a purpose in doing that anyway, unless you either want to allow it to boot homebrew without a memory card (Useful for the late SCPH-90000 series). The region-locking of discs is done by the MECHACON, which can't be reprogrammed.
SCPH-10000 S. MINOKAMO v1.01 (Defunct)
SCPH-10000 S. KISARAZU v1.00 (faulty)
SCPH-15000 S. KOHDA (With warranty seal!) :D
DESR-5100 S. EMCS