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)
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