kconfig: move CONFIG_DEFAULT_DEVICE_TREE to kconfig

This option specifies the default Device Tree used for the run-time
configuration of U-Boot.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Michal Simek <michal.simek@xilinx.com>
diff --git a/dts/Kconfig b/dts/Kconfig
index 20be556..83ba7a6 100644
--- a/dts/Kconfig
+++ b/dts/Kconfig
@@ -45,4 +45,11 @@
 
 endchoice
 
+config DEFAULT_DEVICE_TREE
+	string "Default Device Tree for DT control"
+	help
+	  This option specifies the default Device Tree used for DT control.
+	  It can be overrided from the command line:
+	  $ make DEVICE_TREE=<device-tree-name>
+
 endmenu