This sets a `ip=dhcp` or
`ip=<clientip>:<serverip>:<gatewayip>:<netmaskip>::<iface>:` bootarg for
the network device upon execution of 'ifup'. This is the only point
where we can distinguish between a static ip and a dhcp-based network
setup and thus set a valid bootarg options as it will be required for
nfs boot, for example.

Signed-off-by: Enrico Jorns <ejo@pengutronix.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>