python: new module cffi

Foreign Function Interface for Python calling C code.

[Thomas: add dependency on host-pkgconf and libffi, enable on Python 3
since the module builds fine and loads fine with Python 3 as well.]

Signed-off-by: oli vogt <oli.vogt.pub01@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>