ethernet/arp: Use gateway for non local ipv4 routing.

Detect non local IPv4 destination addresses earlier and route them via
the gw address. Ensure that the ARP table is populated with the GW
address rather than the final destination address.

Jira: ZEP-1473

Change-Id: I3b628584148b760340ef0fea4da4e8893702c832
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>