Fix binary package build

Fixes in build scripts are required (only for master, 2.4 branch should use old variant):
- all `PYTHON_*` environment variables should be renamed to `PYTHON2_*`
- `BUILD_opencv_python` flag -> `BUILD_opencv_python2`
- `opencv_python` target -> `opencv_python2` (for Windows: `modules/python2/opencv_python2`)