Added externally powered USB host mode; added support for usb_audio
This is like OTG, but using an external power source
http://mehrvarz.github.com/usb-host-mode-power-management-nexus7
http://rootzwiki.com/forum/512-nexus-7/ (the most viewed non-pinned thread:
"Can you charge & USB Host mode simultaneously?")

Change-Id: I93136b904fc61091786fc2fe72606d145ccb18ff