Fix XBMC compile error: use internal FFmpeg

PR:2878 (https://github.com/xbmc/xbmc/pull/2878) replaced deprecated CodecID with AVCodecID, but apparently OpenELEC's FFmpeg doesn't provide AVCodecID so disable OpenELEC's FFmpeg and use XBMC's internal FFmpeg.
RetroPlayer: update to 17f3b5f (https://github.com/garbear/xbmc/commit/17f3b5f)
RetroPlayer: update to e670682 (https://github.com/garbear/xbmc/commit/e670682)