net: Add support for simultaneous UDP/TCP and raw sockets

This patch brings support for AF_PACKET and SOCK_RAW type of sockets.
In net_conn_input() function the new flag has been introduced -
'raw_pkt_continue' to indicate if there are other than AF_PACKET
connections registered.

If we do not have other connections than AF_PACKET, the packet is
solely handled in net_conn_input() (or to be more specific in its
helper function - conn_raw_socket()).

Otherwise, it is passed back to net_conn_input in IPv4/6 processing.

Signed-off-by: Lukasz Majewski <lukma@denx.de>