package/opencv3: allow to use pkg-config while cross-compiling

package/opencv3: allow to use pkg-config while cross-compiling

Since version 3.4.1 [1], opencv disable pkg-config while cross-compiling to
avoid host headers/libraries poisoning.

Due to this, opencv fail to detect ffmpeg, gstreamer and gtk dependencies
even if	the corresponding support is requested by
BR2_PACKAGE_OPENCV3_WITH_{FFMPEG,GSTREAMER,GTK2,GTK3}.

Maybe other dependencies are affected by this issue...

While configuring opencv we can notice the following messages:
"-- OpenCV disables pkg-config to avoid using of host libraries. Consider using PKG_CONFIG_LIBDIR to specify target SYSROOT"
"-- Can't find ffmpeg - 'pkg-config' utility is missing"

As the result ffmpeg and gstreamer are not enabled:
--   Video I/O:
--     FFMPEG:                      NO
--       avcodec:                   NO
--       avformat:                  NO
--       avutil:                    NO
--       swscale:                   NO
--       avresample:                NO
--     GStreamer:                   NO
--     libv4l/libv4l2:              NO
--     v4l/v4l2:                    linux/videodev2.h

We can fixes this by adding a new option OPENCV_ENABLE_PKG_CONFIG=ON

--   Video I/O:
--     FFMPEG:                      YES
--       avcodec:                   YES (ver 58.35.100)
--       avformat:                  YES (ver 58.20.100)
--       avutil:                    YES (ver 56.22.100)
--       swscale:                   YES (ver 5.3.100)
--       avresample:                YES (ver 4.0.0)
--     GStreamer:
--       base:                      YES (ver 1.16.0)
--       video:                     YES (ver 1.16.0)
--       app:                       YES (ver 1.16.0)
--       riff:                      YES (ver 1.16.0)
--       pbutils:                   YES (ver 1.16.0)
--     libv4l/libv4l2:              NO
--     v4l/v4l2:                    linux/videodev2.h

Add host-pkgconf in the dependencies.

If there is a path poisoning issue, it will be detected by the toolchain
paranoid wrapper.

[1] https://github.com/opencv/opencv/commit/c4f9ff0285130bd8bcf9d87a9ea6949bdd7e9e5d

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>