The DFU Kconfig menu entries should be part of the SPL
Kconfig file. Also avoid using the top level Makefile by
moving the config dependent build artifacts to the driver/
and driver/usb/gadget/ Makfiles.

With that, DFU can be built again in SPL if
CONFIG_SPL_DFU_SUPPORT is enabled.

Fixes: 6ad61022 ("usb:gadget: Disallow DFU in SPL for now")

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>