ramips: preinit: use iproute2 instead of vconfig

Replace use of vconfig with the corresponding iproute2 syntax in
/lib/preinit/07_set_preinit_iface_ramips

The switch reset work-around needed on some ramips targets used the
deprecated vconfig tool to bring up the pre-init/failsafe interface.
This doesn't work on recent builds where vconfig is no longer being
deployed, resulting in the device being unreachable when triggered
into failsafe mode.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

SVN-Revision: 47349