The system will be hung when udhcpc starts, if nfs is mounted
at "/" directory and default route is different after starting
udhcpc.
The cause is that root_is_nfs() does not work after kernel-2.6.37,
since the device name has been changed from /dev/root to
${IPADDR}:${NFSDIR} on /proc/mounts. which leads to remove
the default routes to nfs server,
Now we use a loose match to check if rootfs is nfs.
