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>

doc/README.fdt-control

@@ -156,6 +156,14 @@ To use this, put something like this in your board header file:
 
 #define CONFIG_EXTRA_ENV_SETTINGS	"fdtcontroladdr=10000\0"
 
+Build:
+
+After board configuration is done, fdt supported u-boot can be build in two ways:
+1)  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
+    $ make
+2)  build the user specified dts file
+    $ make DEVICE_TREE=<dts-file-name>
+
 
 Limitations
 -----------

dts/Makefile

@@ -27,9 +27,11 @@ include $(TOPDIR)/config.mk
 
 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. \