spl: dfu: move DFU Kconfig to SPL Kconfig

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>