This feature, which is disabled by default, extends the neighbor cache
management in order to have multiple IPv6 address associated with a
single link-layer address as neighbor caches.
To use this feature, set 1 to UIP_DS6_NBR_CONF_MAX_NEIGHBOR_CACHES.
nbr_table_update_lladdr() fails to update the lladdr of a nbr with a
new one when the new lladdr is used in nbr_table. This causes
communication errors when NS/NA is employed.
uip_ds6_nbr_update_ll() is introduced to resolve this issue. And
nbr_table_update_lladdr() is removed since it's not used any more.
