busybox: ifupdown:pass interface device name for ipv6 route command

busybox: ifupdown:pass interface device name for ipv6 route command

IPv6 routes need the device argument for link-local routes, or they
cannot be used at all. E.g. "gateway fe80::def" seems to be used in
some places, but kernel refuses to insert the route unless device
name is explicitly specified in the route addition.

Signed-off-by: Haiqing Bai <Haiqing.Bai@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>