x86: Only include cbfs command for coreboot

When running U-Boot bare-metal, the cbfs command is useless.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

include/configs/x86-common.h

@@ -87,7 +87,9 @@
 #define CONFIG_ISO_PARTITION		/* Experimental */
 
 #define CONFIG_CMD_PART
+#ifdef CONFIG_SYS_COREBOOT
 #define CONFIG_CMD_CBFS
+#endif
 #define CONFIG_CMD_EXT4
 #define CONFIG_CMD_EXT4_WRITE
 #define CONFIG_PARTITION_UUIDS