fdt: Avoid emitting an device tree when not needed

U-Boot always needs some sort of a device tree in the build. Some boards
never actually use this, at least in production systems, since a prior
firmware stage sets one up and passes it to U-Boot. At present the only
mechanism to do that is with custom function (OF_BOARD), but future work
will include a standard way of doing this ('standard passage').

It can be confusing to see a device tree emitted from the U-Boot build in
this situation. Add an option to drop it.

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