Add Ethernet hardware MAC address framework to usbnet

Built-in Ethernet adapters support setting the mac address by means of a
ethaddr environment variable for each interface (ethaddr, eth1addr, eth2addr).

This adds similar support to the USB network side, using the names
usbethaddr, usbeth1addr, etc. They are kept separate since we don't want
a USB device taking the MAC address of a built-in device or vice versa.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Eric Bénard <eric@eukrea.com>