net: lwm2m: Retry DTLS handshake before dropping to bootstrap

Allow DTLS handshake to be retried before the engine
drops into bootstrap. Otherwise any termporary failure,
for example DNS failure might drop us into bootstrap
without retrying.

Now all the retry logic should be in sm_do_network_error().
sm_do_registration() should only fall back to bootstrap
if there is configuration error.

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