opencv3: fix Python module build for Python 3.x

opencv3: fix Python module build for Python 3.x

When the OpenCV3 Python support is enabled with Python 3.x, it builds
properly, and the resulting .so file is built for the target
architecture, but its name is wrong:

  output/target/usr/lib/python3.6/site-packages/cv2.cpython-36m-x86_64-linux-gnu.so

This prevents Python 3.x from importing the module:

>>> import cv2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'cv2'

In order to fix this, we simply need to pass PKG_PYTHON_DISTUTILS_ENV
in the environment. The Python module then gets named:

  output/target/usr/lib/python3.6/site-packages/cv2.cpython-36m-arm-linux-gnueabi.so

And can be imported properly:

>>> import cv2
>>>

This solution was suggested by Arnout Vandecappelle in
https://stackoverflow.com/questions/49059035/buildroot-opencv3-python-package-builds-for-the-wrong-target.

With Python 2.x, the module is named just cv2.so so this problem isn't
visible. However, for consistency, we also pass
PKG_PYTHON_DISTUTILS_ENV when building against Python 2.x, by putting
the OPENCV3_CONF_ENV assignment inside the
BR2_PACKAGE_OPENCV3_LIB_PYTHON condition, but outside the
BR2_PACKAGE_PYTHON3/BR2_PACKAGE_PYTHON condition.

Signed-off-by: Sasha Shyrokov <alexander-shyrokov@idexx.com>
[Thomas: extend the commit log, apply the solution to Python 2.x.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>