usb: Fastboot function config for better consistency with other functions

USB download gadget functions such as thor and dfu have a separate config option
for the USB gadget part of the code, independent from the command part.
This switches the fastboot USB gadget to the same scheme, for better
consistency.

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

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