modules: use gmodule-export.
audio: add driver registry, enable module builds.

# gpg: Signature made Mon 12 Mar 2018 10:42:19 GMT
# gpg:                using RSA key 4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/audio-20180312-pull-request:
  audio/sdl: build as module
  audio/pulseaudio: build as module
  audio/oss: build as module
  audio/alsa: build as module
  build: enable audio modules
  audio: add module loading support
  audio: add driver registry
  modules: use gmodule-export

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>