opencv3: use BR2_PACKAGE_PYTHON_NUMPY_ARCH_SUPPORTS

Opencv3 select python-numpy. To avoid duplicating architecture
dependencies, an hidden variable has been added to python-numpy
package. This patch propagate this modification to the opencv3
package.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>