[rbp/omxplayer] Support both analogue and hdmi audio

Thanks to stupid-boy for the original implementation.
[rbp/omxplayer] Add Dynamic Range Compression scheme

This adds a DRC scheme. A number of dBs of amplification can be chosen.
The audio_decode component is setup to do the usual downmix operation early
before most of the audio buffering. This means we get a second or two to
detect if the downmix would overflow. This allows the amplification to be
attenuated in time to avoid the clipping.
The attenuation gradually decays when overflow condition stops.

Also adds a gui setting to boost centre channel.

Requires updated firmware