x86: x86-common.h: Add CONFIG_BOOTDELAY

Without this CONFIG_BOOTDELAY, autobooting does not work at all. As
autoboot_command() from common/* will not get called. So lets define
CONFIG_BOOTDELAY, so that auto-booting works on x86.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Miao Yan <yanmiaobest@gmail.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Tested-by: Miao Yan <yanmiaobest@gmail.com>
Acked-by: Bin Meng <bmeng.cn@gmail.com>

include/configs/x86-common.h

@@ -236,4 +236,6 @@
 	"tftpboot $loadaddr $bootfile;"			\
 	"zboot $loadaddr"
 
+#define CONFIG_BOOTDELAY	2
+
 #endif	/* __CONFIG_H */