usb gadget zero: split out source/sink config

This splits the gadget zero "source/sink" configuration into a standalone
"configuration driver", building on the composite gadget framework code.
It doesn't yet pull the original code out of gadget zero or update how
that driver is built.

Neither this, nor its sibling "loopback" configuration, is a function
driver that can be combined with other functions.  (The host "usbtest"
driver wouldn't know how to deal with that!)  However the code becomes
simpler because of this conversion, so it's a net win.

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