x86: Enable CONFIG_CMD_ZBOOT for coreboot

Enable this option to support booting a zImage.
Signed-off-by: NSimon Glass <sjg@chromium.org>

include/configs/coreboot.h

@@ -39,6 +39,7 @@
 #define CONFIG_LAST_STAGE_INIT
 #define CONFIG_X86_NO_RESET_VECTOR
 #define CONFIG_SYS_VSNPRINTF
+#define CONFIG_ZBOOT_32
 
 /*-----------------------------------------------------------------------
  * Watchdog Configuration
@@ -175,6 +176,8 @@
 #define CONFIG_CMD_FAT
 #define CONFIG_CMD_EXT2
 
+#define CONFIG_CMD_ZBOOT
+
 #define CONFIG_BOOTDELAY	2
 #define CONFIG_BOOTARGS		"root=/dev/mtdblock0 console=ttyS0,9600"