Adds config opt for highbitdepth + misc. vpx

Adds config parameter vp9_highbitdepth, to support highbitdepth profiles.
Also includes most vpx level high bit-depth functions. However
encode/decode in the highbitdepth profiles will not work until
the rest of the code is in place.

Change-Id: I34c53b253c38873611057a6cbc89a1361b8985a6