python: use system libffi for host-python

python: use system libffi for host-python

python2 contains a bundled copy of libffi which is currently out of sync with
the latest libffi release. There is an option to use a system libffi, buildroot
already uses it for the target python2 build and for python3. In python3, the
bundled copy doesn't exist anymore and the system-provided libffi is required.

The bundled copy currently fails to build on aarch64 host due to a missing
definition of AARCH64_CALL_CONTEXT_SIZE. This define was removed from the
headers in recent libffi releases and the host compiler might be including the
system headers before the bundled headers.

To solve this and since buildroot already relies on system libffi for target
python2 and python3 anyway, switch host python2 to use system libffi.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Christian Stewart <christian@paral.in>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>