nodejs: force python interpreter

nodejs build system is based on python, and it does not support python3.

This patch makes sure python2 is used for the build.

Note that, setting PYTHON=... at configure time or in the make
environment is not enough to override all the hard-coded python call, so
we have to sed some python scripts to avoid being screwed.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>