Allow to use python from a local installation

If you want to install ORPSoC and you want it uses
a specific version of python, now you can do:

export PYTHON=/usr/my/python/python2.7
./configure

ORPSoC will use this particular version of python.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Merge pull request #13 from fjullien/python

Allow to use python from a local installation