simple.script/udhcpc: Do not remove someone's else default route

A lease bound/renewal could lead to the removal of a default
route set by pppd. Apply the same "dev $interface" filter that
would be used when using the "route" command.

Fixes: SYS#1163

recipes-core/busybox/files/sysmocom-idu/simple.script

@@ -44,7 +44,7 @@ case "$1" in
 		if [ -n "$router" ] ; then
 			if ! root_is_nfs ; then
                                 if [ $have_bin_ip -eq 1 ]; then
-                                        while ip route del default 2>/dev/null ; do
+                                        while ip route del default dev $interface 2>/dev/null ; do
                                                 :
                                         done
                                 else