x86: Align CBFS on top of ROM

Since the introduction of the new (interim?) master header, coreboot
searches the whole ROM for CBFS entries. Fix that by aligning it on top
of the ROM.

Change-Id: I080cd4b746169a36462a49baff5e114b1f6f224a
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/12810
Tested-by: build bot (Jenkins)
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Reviewed-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>
diff --git a/src/arch/x86/mmap_boot.c b/src/arch/x86/mmap_boot.c
index ae35451..0c2b45dd 100644
--- a/src/arch/x86/mmap_boot.c
+++ b/src/arch/x86/mmap_boot.c
@@ -55,9 +55,8 @@
 
 	props->offset = header.offset;
 	if (CONFIG_ROM_SIZE != header.romsize)
-		props->size = CONFIG_ROM_SIZE;
-	else
-		props->size = header.romsize;
+		props->offset += CONFIG_ROM_SIZE - header.romsize;
+	props->size = CONFIG_ROM_SIZE;
 	props->size -= props->offset;
 	props->size -= header.bootblocksize;
 	props->size = ALIGN_DOWN(props->size, 64);