net: yaip: Add an L2 layer

Net core then does not know anything about l2 related logic.
For instance ARP is used in ethernet l2 API and nowhere else.
This will be helpful when adding different technologies altogether.

Currently, only SLIP driver is enabled to use relevant l2 layer.

Change-Id: I03c93326321028d04222733ca4083e3c6b785202
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>