net: ipv6: Make config options more fine grained

Add CONFIG_NET_IPV6_NBR_CACHE option that is the base for both
neighbor discovery (ND) and duplicate address detection (DAD).

Both ND and DAD can be disabled if needed. If NBR cache is
disabled, then ND and DAD are disabled too.

Note that it makes not much sense to disable DAD or NBR cache
as IPv6 will not work properly without them. It is possible
to disable ND but then the neighbor information needs to come
via other sources like RPL.

Change-Id: I57c8668ad828b3a153dfc58eea78bf5f7ac3938a
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>