Compile USBIP drivers into kernel

1. Create a new directory named usbip in "$(KERNELSOURCE)/drivers/usb/" directory, copy all driver files into that directory. The directory file list below:
   |usbip
   |---- stub.h
   |---- stub_tx.c
   |---- stub_rx.c
   |---- stub_dev.c
   |---- stub_main.c
   |---- usbip_common.h
   |---- usbip_common.c
   |---- usbip_event.c
   |---- vhci.h
   |---- vhci_hcd.c
   |---- vhci_rx.c
   |---- vhci_tx.c
   |---- vhci_sysfs.c
2. Create a Kconfig file in "usbip" directory with contents below:
   #
   # USBIP driver configuration
   #
   comment "Usbip Driver"
   config USBIP
   bool "USBIP support"
   config USBIP_STUB
   tristate "Usbip stub driver"
   depends on USBIP
   config USBIP_VHCI
   tristate "Usbip vhci driver"
   depends on USBIP
3. Modify the Kconfig file of its parent directory "usb", add contents:
   source "drivers/usb/usbip/Kconfig"
   Then we can find a item named "USBIP" in "Device Driver->USB devices support" when we use "make menuconfig/xconfig".
4. Create a Makefile file in "usbip" directory with contents below:
   KSOURCE ?= /lib/modules/$(shell uname -r)/build
   HCD_HEADER:=$(KSOURCE)/drivers/usb/core/hcd.h
   usbip_common_mod-objs    := usbip_common.o usbip_event.o
   usbip-objs         := stub_dev.o stub_main.o stub_rx.o stub_tx.o
   vhci-hcd-objs          := vhci_sysfs.o vhci_tx.o vhci_rx.o vhci_hcd.o
   obj-$(CONFIG_USBIP)    += usbip_common_mod.o
   obj-$(CONFIG_USBIP_STUB)+= usbip.o
   obj-$(CONFIG_USBIP_VHCI)+= vhci-hcd.o
   EXTRA_CFLAGS += -DHCD_HEADER=/"$(HCD_HEADER)/"
5. Modify the Makefile file of its parent directory "usb", add contents:
   obj-$(CONFIG_USBIP)    += usbip/
   All things done!