scons: force host-python dependency to be python2

Scons build-system needs python2 as interpreter (it does not support
python3 yet).

So, we need to force the host dependency to get the python2 interperter
built and available in the host tree to be able to build host-scons
itself and to build scons-based packages, whatever is the python
interpreter for the target.

This patch also makes sure scons will in be called using the right
python interpreter when invoked via $(SCONS).

Fixes:
  http://autobuild.buildroot.net/results/11e6c8c8d79d56ed43daf52e6d6dc80847709926/



[Thomas: take into account the rename of the new variable to
<pkg>_NEEDS_HOST_PYTHON.]

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>