net: ipv6: Allowed to add route to router's link local address

Zephyr doesn't seem to allow on adding route to the device from
which it received Router Advertisement without SLLAO field set
(in particular device that doesn't include Link Layer address).

Changes done:
* Added creating new NBR without Link Layer address set on RA
reception (LL is added only if SLLAO option is set, otherwise
NBR doesn't have LL address, but exists in the table and has
valid IPv6 link local address).
* Removed two asserts preventing from adding route to the NBR
that doesn't have Link Layer address set.

Signed-off-by: Kamil Kasperczyk <kamil.kasperczyk@nordicsemi.no>