samples: mqtt_publisher: wait for the ethernet device to come up

A workaround to allow the network device to come up and settle before
connecting the network.

Fixed a typo on the way and added a boot banner.

Change-Id: I8a8391261610dc00f5168e8abf167c8c65778eae
Signed-off-by: Anas Nashif <anas.nashif@intel.com>