ip: add enable ip4 api

ip: add enable ip4 api

A philosophical question. Do an interface have to have an IPv4 address
to process IPv4 packets? For ICMP error generation it's sufficient that
it has an address available on the node.

More concretely this patch is to allow an extern DHCP client to process
IP packets before it configures an address on the interface, without
having to have an node early in the ip4-unicast feature-arc like
ip4-dhcp-client-detect to intercept the packets.

Type: improvement
Change-Id: I780c579eec28ba564cf8417fbcc87e7a7876fdd2
Signed-off-by: Ole Troan <[email protected]>