package/opencv: add python support

opencv_python module needs python-numpy because it uses some numpy
headers in this wrapper.

>From its 2.4 release, OpenCV offers python bindings, but they required
most of the OpenCV modules to be enabled.

Since OpenCV-3.0.0, python bindings have been reworked:
- it now supports both python2 and python3
- only built modules supporting wrapped in python will be included in
  the bindings.

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