There is not much point in having a network device specific
gateway. If barebox really is part of such a complicated network
in which it needs multiple gateways, then we probably need a
real routing table. Until this happens, a single gateway should
be enough.

This introduces global.net.gateway which holds the gateway ip. The
previously used device specific <ethx>.gateway variables still exist,
but are only aliases for the single gateway.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>