net: Move IPv4 packet handling from net_core.c to ipv4.c

This will refactor net_core.c and move IPv4 specific code
into ipv4.c which is a more logical place for it.

Change-Id: Ia60c5bfec488d2d1a664f113dc3df88e7d5badd1
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>