Revert "package/opencv3: bump to version 4.2.0"

Revert "package/opencv3: bump to version 4.2.0"

This reverts commit 5e51bb2756ee5063eff9a45a46033a449e2a6195.
Indeed, version 4.x is not backward with opencv 3.x, most of the C API
has been removed as stated in https://opencv.org/opencv-4-0.

Moreover, these issues should also be fixed:
 - pkg-config file is not installed by default since
 https://github.com/opencv/opencv/commit/e755a2a6e48ae02dd5136a628cc4148566a08225
 - layout of include files and pkg-config file name are different since
 https://github.com/opencv/opencv/commit/a95673287433fc810eda2d88b94bb234298c4cd5

As a result, ffmpeg fails to build with opencv 4.2.0.

Moreover, it should be noted that -DPROTOBUF_UPDATE_FILES=ON should be
applied in a separate patch to fix existing build failure with 3.4.9 as
stated in https://patchwork.ozlabs.org/patch/1222308

Fixes:
 - http://autobuild.buildroot.org/results/ef1d09d8b234807dcd993422f9557e5c34506013

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>