The cut down version of the task pool has only one task pool, namely,
system task pool. All the task pool API functions accept NULL as a
valid parameter for IotTaskPool_t and use the system task pool when
NULL is passed for the system task pool.
IOT_SYSTEM_TASKPOOL is defined to NULL to use system task pool and
therefore the above assert is no longer valid.
IotTaskPool_GetSystemTaskPool has been removed from the cut down version
of the task pool. define IOT_SYSTEM_TASKPOOL to NULL so that the MQTT library
builds successfully.
Use the new #defines in the iot_config.h.
