python-pynacl: new package

Signed-off-by: Mauro Condarelli <mc5686@mclink.it>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
[Thomas:
 - remove python-six from the build dependencies, it is only a runtime
   dependency
 - add host-python-cffi in the build dependencies.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>