usb: gadget: libcomposite: move config.c into libcomposite

This patch moves config.c into libcomposite and updates all gadgets.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>