[util] Fix up checksum in UNDI ROM header, if present

The UNDI ROM header does contain a checksum byte.  Apparently no-one
cares about this, since iPXE has left it as zero for years without
anyone noticing.

Since Option::ROM now understands the UNDI ROM header, we may as well
fix up the checksum byte for the sake of completeness.

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

src/util/fixrom.pl

   my $image = $rom;
   while ( $image ) {
     $image->pnp_header->fix_checksum() if $image->pnp_header;
+    $image->undi_header->fix_checksum() if $image->undi_header;
     $image->ipxe_header->fix_checksum() if $image->ipxe_header;
     $image->fix_checksum();
     $image = $image->next_image();