opencv: bump to version 2.4.8

This patch bumps OpenCV version and address the minimal set of change to
get the new version building correctly:
- update the download location;
- force Qt version to Qt 4 when support is enabled;
- naively fix the v4l support.

However, it does not address a couple improvements, among these:
- jasper support;
- python/numpy support;
- Qt4 and Qt5 support (since the 2.4.6 release), so only Qt4 support is
  supported in this patch;
- optional libv4l dependency (since 2.4.7 release);
- etc.
These will be addressed in followup patches.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>