[Tutorial] Building an FFmpeg static binary for ARMv7

[bubbleguuum]

BubbleUPnP Server has exactly the same problematic, where it needs a recent ffmpeg compiled with most codecs, and ideally SoX and https support (which are rarely available in stock binaries).

The cool news is that the Optware-ng project provides nice ffmpeg/ffprobe binaries with all of the above for many archs, including all ARM variants and MIPSEL.

These binaries are a breeze to install:

- install Optware-ng following its install instructions
- install the ffmpeg package:
ipkg install ffmpeg
- ffmpeg is in /opt/bin, ready to use

This can be used on Raspberry Pi, Synology (it has a usable default ffmpeg but no ffprobe), QNAP, MIPSEL based routers, i686 Linux, custom devices, etc.