dhcpv6: support a configurable DUID

dhcpv6: support a configurable DUID

Allow the use of a (stable) globally configured DUID as the server identifier.

Currently, odhcpd generates a per-interface DUID-LL, meaning that the DUID is
not stable across interfaces and will change if/when hardware changes.

Supporting a stable DUID brings odhcpd's behaviour in line with RFC8415, §11:

                                                      ...The DUID is
   designed to be unique across all DHCP clients and servers, and stable
   for any specific client or server.  That is, the DUID used by a
   client or server SHOULD NOT change over time if at all possible; for
   example, a device's DUID should not change as a result of a change in
   the device's network hardware or changes to virtual interfaces...

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/274
Signed-off-by: Álvaro Fernández Rojas <[email protected]>