Cross-buildable Debian packaging for LongChair rockchip branches of
FFmpeg and mpv (that is - build .debs on fast x86 machine and try on Rock64).

Current results for me is mpv build that can do hardware decoding of h264 and hevc
on the Rock64 board, and additionally Firefox seemed to also play videos using
the hardware decoding support in the FFmpeg library (although firefox don't
use accelerated output after decoding, which means its too slow when
display resolution is over 1280x720 - use xrandr to change resolution).

Its somewhat tested _only_ with ayufan minimal Debian Stretch arm64 image,
where Rock64 compatible mali, drm, xserver, xf86-video-armsoc and mpp packages
were manually installed. The xf86-video-armsoc was built from Ubuntu Artful sources.
You should probably not use compositing window manager, and use mpv --hwdec=yes
to actually enable the accelerated playback. The drm output is broken,
but x11-egl (named -vo opengl for some reason) works fine at fullscreen
(tested at 1920x1080 resolution with various h264 and h265 videos).

The packaging is less than ideal and many things are probably broken.

Don't try to use this unless you have good knowledge about Debian Linux.