MLK-19697 imx6sx sd board emmc config should use emmc kernel dtb default

change default dtb to imx6sx-sdb-emmc.dtb when config as emmc

Signed-off-by: Frank Li <Frank.Li@nxp.com>
(cherry picked from commit 695f522c07f5cec82e5c53f35fb58c0d763d0054)
diff --git a/include/configs/mx6sxsabresd.h b/include/configs/mx6sxsabresd.h
index 1d28479..4f641ee 100644
--- a/include/configs/mx6sxsabresd.h
+++ b/include/configs/mx6sxsabresd.h
@@ -153,7 +153,7 @@
 		"fi;\0" \
 	"findfdt="\
 		"if test $fdt_file = undefined; then " \
-			"setenv fdt_file imx6sx-sdb.dtb; " \
+			"setenv fdt_file " CONFIG_DEFAULT_DEVICE_TREE ".dtb; " \
 		"fi;\0" \
 
 #define CONFIG_BOOTCOMMAND \