lsxl: unset ncip for rescue mode

Instead of using the serverip we get from the DHCP server, implicitly use
the broadcast address, which is automatically set when no ncip environment
variable is set. That way it isn't necessary to use a special DHCP
configuration to set the netconsole peer.

Signed-off-by: Michael Walle <michael@walle.cc>
Cc: Prafulla Wadaskar <prafulla@marvell.com>