package: rename opencv -> opencv3

package: rename opencv -> opencv3

Since there is a couple of API breaks between OpenCV 2.4 and 3.0, two
distinct packages mutually exclusive will be integrated in the package
tree.

So, this change prepares the re-introduction of the OpenCV-2.4 package
by renaming the current opencv package (which provides OpenCV-3.0) to
opencv3.

Reverse dependencies (vlc) is fixed to use the new symbols.

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:
  - fix missed usage in vlc.mk
  - don't remove legacy OpenCV symbols
  - fix 'endif' comment
  - slightly reword commit log (reverse deps)
]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>