ffmpeg: fix pkg-config utilization

ffmpeg: fix pkg-config utilization

in config.log we can see:
WARNING: aarch64-linaro-linux-pkg-config not found, library detection may fail.

ffmpeg configure script is not looking for pkg-config at the rigt place since it
is assuming cross compilation. let's force its value in the recipe.

This patches 'fixes' library detection, so it also adds:

--disable-libxcb
--disable-libxcb-shm
--disable-libxcb-xfixes
--disable-libxcb-shape

Which were dangling configure options, which started to be enabled after the
pkg-config fix, so they need now to be explicitely disabled. Follow up patch
will enable these options when DISTRO_FEATURES has x11.

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Ross Burton <ross.burton@intel.com>