package/ffmpeg: bump to version 3.1.2

package/ffmpeg: bump to version 3.1.2

- Remove non-existing configure options:
  - --disable-mips32r2
  - --disable-libvo-aacenc
  - --enable-libdcadec
    (https://github.com/FFmpeg/FFmpeg/commit/e259dc86a80398d56d1aabcf6203f80616a4d04d)

- Amend renamed configure options:
  - --disable-mipsdspr1 has been renamed to --disable-mipsdsp

Remove all Kodi-related patches according to upstream repo:
https://github.com/xbmc/FFmpeg/commits/release/3.1-xbmc

Kodi now uses an unpatched ffmpeg.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>