net: lwm2m: add multi fragment support to LwM2M library

The existing LwM2M framework expected contiguous buffers and this
was the reason for the 384 byte buffer sizes.  This was previously
a limitation of the ZoAP API. The new CoAP API doesn't have this
limitation and the LwM2M library has already been migrated to use
it.

Let's finish the process by replacing any contiguous buffer handling
with the correct net_pkt APIs to parse across multiple fragments.

Signed-off-by: Michael Scott <michael.scott@linaro.org>