CONFIG_USB_U_SERIAL (not configurable)

modulename: u_serial.ko

configname: CONFIG_USB_U_SERIAL

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