Add USB host ethernet adapter support

This adds support for using USB Ethernet dongles in host mode. This is just
the framework - drivers will come later. A new config option called
CONFIG_USB_HOST_ETHER can be defined in board config files to switch this
on.

The was originally written by NVIDIA and was cleaned up for release by the
Chromium authors.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 6133160..dc2e3d8 100644
--- a/Makefile
+++ b/Makefile
@@ -235,6 +235,7 @@
 LIBS += drivers/rtc/librtc.o
 LIBS += drivers/serial/libserial.o
 LIBS += drivers/twserial/libtws.o
+LIBS += drivers/usb/eth/libusb_eth.a
 LIBS += drivers/usb/gadget/libusb_gadget.o
 LIBS += drivers/usb/host/libusb_host.o
 LIBS += drivers/usb/musb/libusb_musb.o