package/pkg-waf: use host-python3

This is the last of an unconsolidated series of patches that
removes the need for host-python(2) for all waf-based packages.

Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>