sniper: Fastboot support

This adds support for the fastboot USB gadget, including flashing to the
internal MMC and reboot to bootloader or not.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@konsulko.com>
[trini: Update to use Kconfig for MUSB]
Signed-off-by: Tom Rini <trini@konsulko.com>