Optimize avb_interfaace API implementation.

  This change changes the interface connecting to avb_manager to be a
  smaller core set of functions. The full API is implemented by wrapper
  functions using this core set. This gives a memory win (~4k on the
  current AVB-DC application). In the course of this work there are
  a couple of tidy ups as well.
Merge pull request #1 from davelxmos/opt_interfaces

Optimize avb_interfaace API implementation.