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>