net: lwm2m: Fix blockwise response code

In CoAP blockwise the client is supposed to
respond with 2.31 Continue code on Ack. This was recently
broken when Block1 parsing was moved after the initialization
of reponse packet. We need separate CoAP API to modify the code
of existing CoAP packet.

Also Ack packet should contain the Block1 options, even the
last one.

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