zoap: Add support for retrieving the underlying net_buf

Add an alternative to zoap_packet_get_payload(), that instead of
returning a byte array, returns the net_buf (with the COAP_MARKER
added, if needed) associated with the packet, positioned so the
application can add more data.

Change-Id: I7c955ef42f5ef8406d77da994d1673e6a69b0b6b
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
2 files changed