[depends] move ffmpeg to unified deps
[configure] remove internal ffmpeg
remove ffmpeg from tree
ffmpeg: remove dll wrappers
add ffmpeg lock manager
todo: refactor to own file
ffmpeg: move callbacks into separate file
remove ffmpeg dlls from generated headers
not removing win32 as this might still be needed?
[rpi] setup-sdk.sh: allow overriding of buildroot and xbmc install paths
also don't require root if not needed
add ffmpeg makefile for rpi
sq to rpi - only make xbmc if ffmpeg built successfully