x86: Use PCI setup data

EFI can provide PCI ROMs out of band via boot services, which may not be
available after boot. Add support for using the data handed off to us by
the boot stub or bootloader.

[bhelgaas: added Seth's boot_params section mismatch fix]
[bhelgaas: drop "boot_params.hdr.version < 0x0209" test]
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Seth Forshee <seth.forshee@canonical.com>