Implement FunctionFS transport

Implement FunctionFS transport

FunctionFS is a generic interface between the kernel and userland
processes that allows implementation of USB functions as userspace
processes. There may be many instances of FunctionFS mounted each
one for a different USB functions.

Authored-by: Łukasz Stelmach <l.stelmach@samsung.com>
[Backported from other repository]
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
[Rebased]
Signed-off-by: Paweł Szewczyk <p.szewczyk@samsung.com>
Change-Id: Idba4fd657006d445bf1399218784f1c40e3cf626