usb gadget zero: use composite gadget framework

Update Gadget Zero to use the more modular versions of the loopback
and source/sink configuration drivers which build on the new gadget
framework code.

The core code is a LOT simpler, and it should be much easier now to
understand how the parts fit together.  The conversion is an overall
source shrink in terms of this gadget, since it uses more midlayer
support.  However, it's an overall increase in object size because
there's less sharing between the two configurations (improves code
clarity) and because the midlayer is a bit more functional than this
driver actually needs.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>