spl: add USB Gadget config option

Introduce USB Gadget config option. This allows to combine Makefile
entries for SPL_USBETH_SUPPORT and SPL_DFU_SUPPORT.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Ravi Babu <ravibabu@ti.com>
5 files changed