configs: x86: allow to override CONFIG_BOOTCOMMAND

Allow to override CONFIG_BOOTCOMMAND in .config.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/include/configs/x86-common.h b/include/configs/x86-common.h
index 994214e..e8f680f 100644
--- a/include/configs/x86-common.h
+++ b/include/configs/x86-common.h
@@ -62,8 +62,10 @@
  * Command line configuration.
  */
 
+#ifndef CONFIG_BOOTCOMMAND
 #define CONFIG_BOOTCOMMAND	\
 	"ext2load scsi 0:3 01000000 /boot/vmlinuz; zboot 01000000"
+#endif
 
 #if defined(CONFIG_CMD_KGDB)
 #define CONFIG_KGDB_BAUDRATE			115200