net: lwm2m: Use CID_SUPPORTED instead of CID_ENABLED

When ENABLED flag is used, we generate 32 byte DTLS
Connection Identifier and include that in our
DTLS Client HELO. This has no benefit as client only
has one connection toward the server, it does not need
any identification.

When SUPPORTED flag is used, we just include
zero length Connection Identifier in the handshake,
which tell server that we support Connection Identifier
and server can generate one for it.
We then use the CID in the packets that we send towards
server, but response packets don't contain any CID.
This gives all the benefit of CID as server is able to
identify us even when NAT mapping have changed.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>