ARM: at91: sama5: move the sd/mmc env config to at91-sama5_common.h

As almost all sama5 sd/mmc env configurations are same, so move them to
at91-sama5_common.h.

Also define CONFIG_ENV_VARS_UBOOT_CONFIG to have the varaible: $board_name.
Then we can use 'at91-${board_name}.dtb' as the dtb name.

TODO: since sama5d3xek has different dtb name, we need to some extra
stuff to make it work on sama5d3xek boards.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
diff --git a/include/configs/sama5d3_xplained.h b/include/configs/sama5d3_xplained.h
index c4141a0..91ef123 100644
--- a/include/configs/sama5d3_xplained.h
+++ b/include/configs/sama5d3_xplained.h
@@ -124,15 +124,7 @@
 				"nand read 0x22000000 0x200000 0x600000;" \
 				"bootz 0x22000000 - 0x21000000"
 #elif CONFIG_SYS_USE_MMC
-/* bootstrap + u-boot + env in sd card */
-#define CONFIG_ENV_IS_IN_FAT
-#define FAT_ENV_INTERFACE	"mmc"
-#define FAT_ENV_FILE		"uboot.env"
-#define FAT_ENV_DEVICE_AND_PART	"0"
-#define CONFIG_ENV_SIZE		0x4000
-#define CONFIG_BOOTCOMMAND	"fatload mmc 0:1 0x21000000 at91-sama5d3_xplained.dtb; " \
-				"fatload mmc 0:1 0x22000000 zImage; " \
-				"bootz 0x22000000 - 0x21000000"
+/* override the bootcmd, bootargs and other configuration for sd/mmc env */
 #else
 #define CONFIG_ENV_IS_NOWHERE
 #endif