Implement logic to perform IPv6 SLAAC in GTPU tunnel
The IPv6 SLAAC prcoedure is not yet triggered in this patch; it will be
triggered by cups_client's new command in follow-up patch.
Depends: libosmo-netif.git Change-Id I78ec7270c717af0a1b8ffd9398cd69ea7a0dbee2
Change-Id: I837a4d7ec7c134412ab4a2e09909670e81ecbeea
