sandbox: Build a device tree file for sandbox

Add support for building a device tree for sandbox's CONFIG_OF_HOSTFILE
option to make it easier to use device tree with sandbox.

This adjusts the Makefile to build a u-boot.dtb file which can be passed
to sandbox U-Boot with:

   ./u-boot -d u-boot.dtb

Signed-off-by: Simon Glass <sjg@chromium.org>