boost: make build with python an option

Add an option to boost to build the Boost.Python library.
http://www.boost.org/doc/libs/1_55_0/libs/python/doc/index.html



Signed-off-by: Johan Derycke <johan.derycke@barco.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>