Add DT and Kconfig overlay for new experimental CDC ECM
implementation. USB device support configuration and
initialization can be done through shell by following
commands:
uart:~$ usbd config add 1
uart:~$ usbd class add cdc_ecm_0 1
uart:~$ usbd defaults
uart:~$ usbd init
uart:~$ usbd enable
Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>
