x86: Build a .rom file which can be flashed to an x86 machine

On x86 machines U-Boot needs to be added to a large ROM image which is
then flashed onto the target board. The ROM has a particular format so it
makes sense for U-Boot to build this image automatically. Unfortunately
it relies on binary blobs so we cannot require this for the default
build as yet.

Create a u-boot.rom output file for this purpose.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/chromebook_link.h b/include/configs/chromebook_link.h
index 394e27b..3137ef0 100644
--- a/include/configs/chromebook_link.h
+++ b/include/configs/chromebook_link.h
@@ -18,8 +18,10 @@
 #define CONFIG_SYS_CAR_ADDR			0xff7e0000
 #define CONFIG_SYS_CAR_SIZE			(128 * 1024)
 #define CONFIG_SYS_MONITOR_LEN			(1 << 20)
+#define CONFIG_SYS_X86_START16			0xfffff800
 #define CONFIG_BOARD_EARLY_INIT_R
 
+#define CONFIG_X86_RESET_VECTOR
 #define CONFIG_NR_DRAM_BANKS			8
 
 #define CONFIG_COREBOOT_SERIAL