CONFIG_USB_LIBCOMPOSITE (not configurable)

modulename: libcomposite.ko

configname: CONFIG_USB_LIBCOMPOSITE

Linux Kernel Configuration
└─>Device Drivers
└─>USB support
└─>USB Gadget Support
└─>USB_LIBCOMPOSITE
In linux kernel since version 3.1 (release Date: 2011-10-24)  
source code:
drivers/usb/gadget/