[build] Add flags target that outputs our `$(CFLAGS)`
[io][posix_asynch] Provide lwIP compatibility and add copyright headers
[fix][build] Should not touch `$(XI_CFLAGS)` directly at this point
[fix] MQTT requires the QoS level of the subscribe packet to be 1, not 2
Merge pull request #116 from xively/subscribe_qos_fix

[fix] MQTT requires the QoS level of the subscribe packet to be 1, not 2
[fix] first part of improvements towards better error handling

* tasks now contain their own coroutine states so they can be used in qos 1 and 2
* fixed the qos subscribe level should be at least once