net: Move IPv6 packet handling from net_core.c to ipv6.c

This will refactor net_core.c and move IPv6 specific code
into ipv6.c which is a more logical place for it.

Change-Id: I5bbecbb760111326b9a6bbef5802c53d7f6efda9
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>