python-cffi: add host variant

Host variant is needed to cross-compile CFFI based C library wrappers.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
[Thomas:
 - add dependency on host-pkgconf for the host package variant.
 - add various environment variables needed to make pkg-config behave
   properly when building host-python-cffi. Otherwise, pkg-config
   returns values that are appropriate to build things for the target,
   and the build fails.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>