How to enable ethernet over usb in U-Boot?

Hello, everyone.

I want to use tftp in U-Boot, my board is BBGW, so I want to use ethernet over usb to talk to my host.

The following is my development environment.

windows 10 (host)
ubuntu 18 runs on VirtualBox (guest)

My steps,

  1. enter uboot

  2. setenv ipaddr

  3. setenv serverip

  4. setenv ethact usb_ether

  5. setenv usbnet_devaddr f8:dc:7a:00:00:02

  6. setenv usbnet_hostaddr f8:dc:7a:00:00:01

  7. saveenv

Then I have no idea how can I set a RNDIS network interface like offical driver does? may be helpful.