Framework for building codecs outside of Rockbox

[saratoga]

Currently FLAC can build separately from Rockbox because it has a separate parser that uses standard c functions instead of rockbox API functions.  If instead wrappers were written around the various playback calls (ex: ci->read_filebuf ==> read()), then we could compile stand alone versions of all the codecs.

This would be beneficial since it would enable easier debugging of codecs and parsers, would open up the possibility of profiling/debugging codecs on arm/linux emulators without needing to emulate DAP hardware, and would allow other projects that want to use our codecs to more easily include them.