Improve token handling by removing special meaning of tokenlen == 0,
which allows to handle server requests w/o a token (so far such
requests would cause the lwm2m engine to autogenerate token in the
response).
In order to autogenerate token during message initialization, use
special symbol `LWM2M_MSG_TOKEN_GENERATE_NEW`. If no token is wished to
be used, simply set the tokenlen to 0.
Additionally, fix an issue with token autogeneration, where invalid
token len was used (0 instead of 8).
Fixes#28299
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
2497958298