bsd.pypy.cffi.mk: unbreak building of cffi ports

To fix conflicting of pypy and pypy3 the PYPY_DIR value was set using
${PORTNAME}, however the same codepath is used for cffi ports who's
${PORTNAME} is not pypy or pypy3.

Fix this by using an intermediate PYTHON_IMPL variable that is conditionally
set to ${PORTNAME} and explicitly set to 'pypy' for cffi ports.

Reported by: pkg-fallout