[OE-core] proposal: let's move oe-core to ffmpeg (from libav)

[Alexander Kanavin]

Hello,

let's replace libav with ffmpeg in oe-core. The reasons are:

- all of the major distributions are using ffmpeg now (the last holdout 
was Debian, and they switched to ffmpeg in summer 2015 after a long 
discussion and deliberation [1][2])
- ffmpeg is much more responsive to security issues
- ffmpeg does regular releases, while libav hasn't had one since spring 2015
- other reasons listed in [2]

[1] https://lwn.net/Articles/650816/
[2] https://wiki.debian.org/Debate/libav-provider/ffmpeg

Objections, thoughts?


Alex