rockchip: utilize CONFIG_DEFAULT_FDT_FILE

Currently the fdtfile environment variable is set to
CONFIG_DEFAULT_DEVICE_TREE which is Ñ–nternally used as U-Boot devicetree
source. The OS can use a different filename and Kconfig gives us the
ability to select a default devicetree via CONFIG_DEFAULT_FDT_FILE.
This also gives user configuring U-Boot via menuconfig the behaviour
someone would expect.

Signed-off-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Tested-By: Vagrant Cascadian <vagrant@debian.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/include/configs/rk3128_common.h b/include/configs/rk3128_common.h
index cb07466..94b0ae0 100644
--- a/include/configs/rk3128_common.h
+++ b/include/configs/rk3128_common.h
@@ -53,6 +53,7 @@
 #include <config_distro_bootcmd.h>
 #define CONFIG_EXTRA_ENV_SETTINGS \
 	ENV_MEM_LAYOUT_SETTINGS \
+	"fdt_file=" CONFIG_DEFAULT_FDT_FILE "\0" \
 	"partitions=" PARTS_DEFAULT \
 	BOOTENV