./ffmpeg -fflags +genpts -hwaccel vaapi -hwaccel_output_format vaapi -vaapi_device /dev/dri/card0 -i file:"/somefile.mkv" -map 0:0 -map 0:1 -map -0:s -codec:v:0 h264_vaapi -force_key_frames "expr:gte(t,n_forced*5)" -vf "format=nv12|vaapi,hwupload" -b:v 4783434 -maxrate 4783434 -bufsize 9566868 -vsync -1 -level 41 -map_metadata -1 -threads 0 -codec:a:0 aac -strict experimental -ac 6 -ab 192000 -af "aresample=async=1" -f mp4 -movflags frag_keyframe+empty_moov -y "/someotherfile.mp4"
ffmpeg version N-81820-gdba2db6-static http://johnvansickle.com/ffmpeg/  Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 5.4.1 (Debian 5.4.1-2) 20160904
  configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --enable-libmp3lame --enable-libx264 --enable-libx265 --enable-libwebp --enable-libspeex --enable-libvorbis --enable-libvpx --enable-libfreetype --enable-fontconfig --enable-libxvid --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libtheora --enable-libvo-amrwbenc --enable-gray --enable-libopenjpeg --enable-libopus --enable-libass --enable-gnutls --enable-libvidstab --enable-libsoxr --enable-frei0r --enable-libfribidi --disable-indev=sndio --disable-outdev=sndio --enable-librtmp --enable-libzimg --cc=gcc-5 --disable-ffplay
  libavutil      55. 32.100 / 55. 32.100
  libavcodec     57. 60.100 / 57. 60.100
  libavformat    57. 51.100 / 57. 51.100
  libavdevice    57.  0.102 / 57.  0.102
  libavfilter     6. 63.100 /  6. 63.100
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  2.100 /  2.  2.100
  libpostproc    54.  0.100 / 54.  0.100
[AVHWDeviceContext @ 0x44c2720] No VA display found for device: /dev/dri/card0.
[vaapi @ 0x308c1e0] Failed to create a VAAPI device