package/kodi: switch to internal ffmpeg version 3.1.11 build

package/kodi: switch to internal ffmpeg version 3.1.11 build

Kodi 17 is incompatible with ffmpeg-4.x. To prepare the ffmpeg bump we
switch the current Kodi package to internally build and statically link
to patched ffmpeg-3.1.11 provided by upstream. Gnutls is added as
dependency to allow playback of https streams.

Upstream expects Kodi 17 to be used with ffmpeg 3.1.x (see upstream PR
12368) so we choose the upstream way to build ffmpeg instead of provi-
ding a version choice for ffmpeg in buildroot.

This commit can be reverted when Kodi is bumped to version 18, currently
released as alpha3.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>