net: Use network driver when sending IP packets

Set the uIP output function in order to call the send function in the
registered driver. Do driver initialization and checking cleanups.

Change-Id: I18c9b551320d15fa2619a077d1224f5e521c53ed
Signed-off-by: Patrik Flykt <patrik.flykt@linux.intel.com>
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>