RELOCATABLE_RAMSTAGE: Fix weak symbols with option ROMs

After relocation the weak symbol map_oprom_vendev is no longer NULL.
Always have empty stub function defined.

Change-Id: I5b1bdeb3f37bb04363cf3d9dedaeafc9e193aaae
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/7956
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
diff --git a/src/include/device/pci_rom.h b/src/include/device/pci_rom.h
index fe77276..a929206 100644
--- a/src/include/device/pci_rom.h
+++ b/src/include/device/pci_rom.h
@@ -35,6 +35,6 @@
 
 struct rom_header *pci_rom_probe(struct device *dev);
 struct rom_header *pci_rom_load(struct device *dev, struct rom_header *rom_header);
-u32 __attribute__((weak)) map_oprom_vendev(u32 vendev);
+u32 map_oprom_vendev(u32 vendev);
 
 #endif