ffmpeg: rework

[awiouy]

I wanted to know how building FFmpeg for LibreELEC compared to a vanilla FFmpeg cross compilation. I think this is it.

The only update is that --enable-pic was added for aarch64.

The other changes should be cosmetic:

• removed the default and autodetected build options
• removed superfluous variables (FFMPEG_CPU, FFMPEG_FPU and FFMPEG_PIC)
• split the code into logic for dependencies and logic for pre_configure_target.

The output of the configure step is the same as with the previous version of ffmpeg for Generic and RPi2. We could maybe use Jenkins to check the other builds

[awiouy]

This is now the first step of a rework of the ffmpeg package.

Changes are cosmetic and include

• systematic use of quotes
• remove unset variable FFMPEG_CPU
• set systematically --enable-pic, instead of inconsistent use of --enable-pic and -fPIC
• remove useless setting of FFMPEG_FPU=""
• remove strip_lto and strip_gold, default stripping appears to work fine on RPi2
• remove inconsistent setting of --extra-version
• remove useless setting of --build-suffix
• export is not required
• explicity --disable-neon and --disable-hardcoded-tables when they do not apply

Please merge

[lrusak]

thanks!