package/opencv4: new package

package/opencv4: new package

opencv4 is not fully backward compatible with opencv3 as "a lot of C API
from OpenCV 1.x has been removed.": https://opencv.org/opencv-4-0.

Moreover, it should be noted that layout of include files as well as
pkg-config filename changed since
https://github.com/opencv/opencv/commit/a95673287433fc810eda2d88b94bb234298c4cd5

All these changes mean that most of buildroot packages (e.g. vlc,
ffmpeg) are not compatible with opencv4.

Here is the most important changes between opencv3 and opencv 4.5.1:
 - License is Apache-2.0 (https://opencv.org/opencv-4-5-0)
 - openjpeg is an optional dependency (enabled by default) since
   https://github.com/opencv/opencv/commit/0384eb7d8c1a31831704e31b8bdbfbc6ff854281
   jpeg2000 can be enabled through openjpeg or jasper
 - pkg-config must be enabled with OPENCV_GENERATE_PKGCONFIG since
   https://github.com/opencv/opencv/commit/e755a2a6e48ae02dd5136a628cc4148566a08225
 - cmake is not installed in /usr/share anymore so hook can be dropped
 - C++11 is required since
   https://github.com/opencv/opencv/commit/d4688e6474dcd59c1d66736b744a8049f386a90b

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Arnout: remove a few redundant dependencies from Config.in]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>