package/opencv3: ensure the python module works when BR2_PACKAGE_PYTHON{, 3}_PYC_ONLY=y

package/opencv3: ensure the python module works when BR2_PACKAGE_PYTHON{, 3}_PYC_ONLY=y

The OpenCV Python module does a fairly strange thing to read a few
configuration details: it uses Python's execfile() to execute two .py
files and access a few variables. However, execfile() only works with
.py files and not .pyc files.

When BR2_PACKAGE_PYTHON{,3}_PYC_ONLY=y, the .py files are all removed,
causing the OpenCV Python module to not work:

  File "usr/lib/python3.7/site-packages/cv2/__init__.py", line 89, in <module>
  File "usr/lib/python3.7/site-packages/cv2/__init__.py", line 58, in bootstrap
  File "usr/lib/python3.7/site-packages/cv2/__init__.py", line 56, in load_first_config
ImportError: OpenCV loader: missing configuration file: ['config.py']. Check OpenCV installation.

To fix this problem, this commit uses the newly introduced
<pkg>_KEEP_PY_FILES mechanism, to ensure the important config*.py
files are kept.

Fixes:

  https://bugs.busybox.net/show_bug.cgi?id=12171

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>