package/opencv: re-introduce opencv for opencv-2.4

As Jonathan noticed in [1], users' applications may depend on opencv-2.4
APIs removed in opencv-3.0.

So, re-introduce opencv package as it was right before the bump to
opencv-3.0 (i.e.: commit bf00b5a9).

We do not support both OpenCV-2.4 and OpenCV-3 at the same time, so make
OpenCV-3 depend on !OpenCV-2.4.

[1] http://lists.busybox.net/pipermail/buildroot/2015-August/135270.html



Cc: Jonathan Ben Avraham <yba@tkos.co.il>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
[yann.morin.1998@free.fr:
  - remove legacy symbols, now
  - make opencv3 depends on !opencv, not the other way around
  - slitghly reword the commit log (opencv/opencv3 dependency)
]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>