ARM: at91: sama5d3xek: use a $dtb_name to load dtb

Since sama5d3xek boards has different type of dtb blobs, so we need to detect
the cpu type in runtime.

So we add a new variable $dtb_name. if $dtb_name is not defined, we just use
	at91-${board_name}.dtb

as the $dtb_name. Otherwise, we will just load the dtb with
	$dtb_name.

For sama5d3xek, we will detect cpu type and make up $dtb_name in
runtime.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
diff --git a/include/configs/at91-sama5_common.h b/include/configs/at91-sama5_common.h
index bb737a1..0fe0852 100644
--- a/include/configs/at91-sama5_common.h
+++ b/include/configs/at91-sama5_common.h
@@ -61,7 +61,10 @@
 #define FAT_ENV_FILE		"uboot.env"
 #define CONFIG_ENV_SIZE		0x4000
 
-#define CONFIG_BOOTCOMMAND	"fatload mmc 0:1 0x21000000 at91-${board_name}.dtb; " \
+#define CONFIG_BOOTCOMMAND	"if test ! -n ${dtb_name}; then "	\
+				    "setenv dtb_name at91-${board_name}.dtb; " \
+				"fi; "					\
+				"fatload mmc 0:1 0x21000000 ${dtb_name}; " \
 				"fatload mmc 0:1 0x22000000 zImage; "	\
 				"bootz 0x22000000 - 0x21000000"
 #define CONFIG_BOOTARGS							\