iot: Add MQTT v3.1.1 packet handling support for Zephyr

This commit adds support for the MQTT protocol v3.1.1.
Specifically, this commit allows a Zephyr application to create
the following MQTT messages:

CONNACK, CONNECT, PUBLISH, PUBACK, PUBREC, PUBREL, PUBCOMP, UNSUBACK,
SUBSCRIBE, SUBACK, UNSUBSCRIBE, PINGREQ, PINGRESP, and DISCONNECT.

Furthermore, the following messages can be parsed by the routines
provided by this commit:

CONNACK, CONNECT, PUBLISH, PUBACK, PUBREC, PUBREL, PUBCOMP, UNSUBACK,
SUBSCRIBE, SUBACK, PINGREQ, PINGRESP and DISCONNECT.

NOTE: client behavior (routines with network access) and QoS will be
integrated in future patches.

The MQTT v3.1.1 specification can be found at:

  http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/mqtt-v3.1.1.html

Origin: Original

Jira: ZEP-365
Jira: ZEP-591
Jira: ZEP-856

Change-Id: Ie0c179370cea22f7554564692bc426a8d5c419d2
Signed-off-by: Flavio Santes <flavio.santes@intel.com>
8 files changed