init: Add symlinks to USB character devices

Since we now support USB host mode, devices might get a different device
file depending if other devices are connected on the host port. This
patch creates symlinks to USB character devices that depend on the port
devices are connected to, but not on the order they have been probed.
(e.g. /dev/usb/tty2-1:1.0 -> /dev/ttyUSB0)

Change-Id: I285bcc2d59446fbff8a7abbe39c21781f3bb4bd3