Audio: MFCC: Add Voice Activity Detection based on Mel spectrum

[singalsu]

This patch adds a new mfcc_vad module that implements VAD operating on the Mel log spectrum values produced by the MFCC component. The VAD is very simple and is not very selective for voice vs. other signals. But the continuously updated background noise estimate prevents stationary noises to trigger the VAD.

The algorithm tracks a per-bin noise floor (instant-down, slow-rise) and computes a A-weighted energy delta. The used weight emphasizes speech frequencies. Speech is declared when the delta exceeds a threshold (0.30 in Q9.23) with a 20-frame hangover to prevent rapid toggling.

The VAD flag is inserted into the output stream as the first value after the magic header word in all format paths (S16, S24, S32).

A new Kconfig option CONFIG_COMP_MFCC_VAD (depends on COMP_MFCC, default n) gates compilation of the VAD code and the stream format change.

[singalsu]

This is still WIP. I'd like to add a better audio feature header to the fake PCM stream. In successive PRs should start to use the compress PCM type for MFCC output data. The MFCC config blob could enable for VAD mode discontinuous data. E.g. once per second background noise Mel spectrum values, for speech detected at FFT hop rate, e.g. every 10 ms.