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/whistler_defconfig b/configs/whistler_defconfig
index 8c07c18..9553eb8 100644
--- a/configs/whistler_defconfig
+++ b/configs/whistler_defconfig
@@ -2,3 +2,4 @@
 +S:CONFIG_TEGRA=y
 +S:CONFIG_TEGRA20=y
 +S:CONFIG_TARGET_WHISTLER=y
+CONFIG_DEFAULT_DEVICE_TREE="tegra20-whistler"