init: select usb gadget controller at run time

Right now we set sys.usb.controller property in init.$platform.usb.rc
to enable ConfigFS gadgets.

Let system detect and set UDC driver name from /sys/class/udc instead.

Change-Id: I99ad49d24dc53b543c5573e209bf00d6c2d91dd4
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>