mx51evk: savenv or env save command does not work

fix saveenv or env save command not work on mx51evk board.
with this patch, we can use savenv or env save to
store enviroments to mmc card slot 0
Signed-off-by: NJason Liu <r64343@freescale.com>

include/configs/mx51evk.h

@@ -216,8 +216,9 @@
  */
 #define CONFIG_SYS_NO_FLASH
 
-#define CONFIG_ENV_SECT_SIZE    (128 * 1024)
-#define CONFIG_ENV_SIZE		CONFIG_ENV_SECT_SIZE
-#define CONFIG_ENV_IS_NOWHERE
+#define CONFIG_ENV_OFFSET      (6 * 64 * 1024)
+#define CONFIG_ENV_SIZE        (8 * 1024)
+#define CONFIG_ENV_IS_IN_MMC
+#define CONFIG_SYS_MMC_ENV_DEV 0
 
 #endif