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/sama5d3xek.h b/include/configs/sama5d3xek.h
index 5c7b722..7b2f69a 100644
--- a/include/configs/sama5d3xek.h
+++ b/include/configs/sama5d3xek.h
@@ -19,6 +19,9 @@
  */
 #include "at91-sama5_common.h"
 
+#define CONFIG_BOARD_LATE_INIT
+#define CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG
+
 /* serial console */
 #define CONFIG_ATMEL_USART
 #define CONFIG_USART_BASE		ATMEL_BASE_DBGU