configure: allow building with python3

configure: allow building with python3

Pretty much all of the scripts are python2+3 compatible.
Check and allow using python3, while adjusting the PYTHON2 refs.

Note:
 - python3.4 is used as it's the earliest supported version
 - python2 chosen prior to python3

v2: use python2 by default

Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>