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/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 84681db..47d8400 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -1,2 +1,3 @@
 CONFIG_OF_CONTROL=y
 CONFIG_OF_HOSTFILE=y
+CONFIG_DEFAULT_DEVICE_TREE="sandbox"