Add CDC Ethernet Emulation Model function driver. This usb network
function can be used for ethernet over USB. Ethernet packet are
encapsulated within EEM packets. An EEM pkt contains 2-byte header
and 4-byte sentinel (or crc).
Note that EEM packets can be split across USB packets but shall not
be split across USB transfers.
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>