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/snow_defconfig b/configs/snow_defconfig
index 2d59046..14ed793 100644
--- a/configs/snow_defconfig
+++ b/configs/snow_defconfig
@@ -2,3 +2,4 @@
 +S:CONFIG_ARM=y
 +S:CONFIG_ARCH_EXYNOS=y
 +S:CONFIG_TARGET_SNOW=y
+CONFIG_DEFAULT_DEVICE_TREE="exynos5250-snow"