The Option::ROM module now compares the Code Type in the PCIR header
to 0x00 (PC-AT) in order to check the presence of other header types
(PnP, UNDI, iPXE, etc).  The validity of these headers are checked not
only by offset, but by range and signature checks also.  The image
checksum and initial size also depends on Code Type.

Signed-off-by: Michael Brown <mcb30@ipxe.org>