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>