[util] Update mergerom.pl to handle iPXE ROM header

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

src/util/mergerom.pl

 
 foreach my $rom ( @roms ) {
 
-  # Update base length
-  $baserom->{length} += $rom->{length};
-
   # Merge initialisation entry point
   merge_entry_points ( $baserom->{init}, $rom->{init}, $offset );
 
     merge_entry_points ( $baserom_pnp->{bev}, $rom_pnp->{bev}, $offset );
   }
 
+  # Update iPXE header, if present
+  my $baserom_ipxe = $baserom->ipxe_header;
+  my $rom_ipxe = $rom->ipxe_header;
+  if ( $baserom_ipxe ) {
+
+    # Update shrunk length
+    $baserom_ipxe->{shrunk_length} = ( $baserom->{length} +
+				       ( $rom_ipxe ?
+					 $rom_ipxe->{shrunk_length} :
+					 $rom->{length} ) );
+
+    # Fix checksum
+    $baserom_ipxe->fix_checksum();
+  }
+
+  # Update base length
+  $baserom->{length} += $rom->{length};
+
   # Fix checksum for this ROM segment
   $rom->fix_checksum();