dts/Makefile: Build the user specified dts

This patch provides a support to build the user specified dts.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/dts/Makefile b/dts/Makefile
index 6f36153..03e163e 100644
--- a/dts/Makefile
+++ b/dts/Makefile
@@ -27,9 +27,11 @@
 
 LIB	= $(obj)libdts.o
 
+ifeq ($(DEVICE_TREE),)
 $(if $(CONFIG_DEFAULT_DEVICE_TREE),,\
 $(error Please define CONFIG_DEFAULT_DEVICE_TREE in your board header file))
 DEVICE_TREE = $(subst ",,$(CONFIG_DEFAULT_DEVICE_TREE))
+endif
 
 $(if $(CONFIG_ARCH_DEVICE_TREE),,\
 $(error Your architecture does not have device tree support enabled. \