x86: config: Enable dhcp on link

The dhcp option is required to get bootp to work on the Chromebook Pixel,
so enable it.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/configs/coreboot.h

@@ -297,6 +297,12 @@
 #define CONFIG_USB_HOST_ETHER
 #define CONFIG_USB_ETHER_ASIX
 #define CONFIG_USB_ETHER_SMSC95XX
+#define CONFIG_TFTP_TSIZE
+#define CONFIG_CMD_DHCP
+#define CONFIG_BOOTP_BOOTFILESIZE
+#define CONFIG_BOOTP_BOOTPATH
+#define CONFIG_BOOTP_GATEWAY
+#define CONFIG_BOOTP_HOSTNAME
 
 #define CONFIG_CMD_USB