package/opencv: disable all modules by default

This change allows to reduce the size of the default OpenCV package, so
the final root filesystem.

However, this will break any existing defconfig enabling OpenCV. Indeed,
a minimal OpenCV  package will be built, whereas, prior to applying this
patches, a full-featured one would have been built instead.

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