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/doc/README.fdt-control b/doc/README.fdt-control
index 85bda03..8352835 100644
--- a/doc/README.fdt-control
+++ b/doc/README.fdt-control
@@ -156,6 +156,14 @@
 
 #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
 -----------