kernel: add option for USB FunctionFS gadget

kernel: add option for USB FunctionFS gadget

Add menuconfig option 'kmod-usb-gadget-fs' for the USB FunctionFS gadget
module

OpenWRT is perfect for USB gadgets that are network-enabled, since it
provides everything needed to easily manage the device. Having support
for the FunctionFS gadget enables the use of any custom USB devices
that does not have a specific module, like Media Transfer Protocol.

Signed-off-by: Dominic Germain <dominic@germain.cc>
Link: https://github.com/openwrt/openwrt/pull/21207
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>