zed-audio: Set ADAU1761 codec as default. Fixup hardcoded paths.

Remove the /etc and /lib paths from the install script, use variables.
Add an "asound.conf" file that picks the audio codec as default output.
my-image: Improve post-processing

Run populate-volatile on the host. Do not change working directory in the
hook script.
An image to demonstrate MP3 playback

Add mp3-image that installs the things you need to turn your Zynq
board into an MP3 player. Add licensing exception to the distro
configuration to allow installing the codecs.