net: Add initial IP stack offloading support

If IP stack offloading is enabled, then call corresponding
L2 driver when corresponding net_context API function is
called. The L2 driver can then do its magic to send/recv
IP packet or do what ever with it.

Change-Id: I26695cee6feb80a41923db0165f22d94477333be
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>