usb: gadget: Weak board_usb_init/cleanup definitions in USB download gadget code

Weak versions of board_usb_init and board_usb_cleanup are defined in common USB
host code, but it is also used for USB device gadgets, so we also need a weak
definition of it when there is no USB host enabled.
Both weak definitions do not conflict.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>

Test HW: Odroid_XU3 (Exynos5422), trats (Exynos4210)