Use ip to setup the gateway in sysinit.go

ip from iproute2 replaces the legacy route tool which is often not
installed by default on recent Linux distributions.

The same patch has been done in network.go and is re-used here.
Upstream-commit: 5e1a975b48c1d4e6743f68ca770f8fe06fb32ce4
Component: engine

components/engine/sysinit.go

@@ -17,8 +17,7 @@ func setupNetworking(gw string) {
 	if gw == "" {
 		return
 	}
-	cmd := exec.Command("/sbin/route", "add", "default", "gw", gw)
-	if err := cmd.Run(); err != nil {
+	if _, err := ip("route", "add", "default", "via", gw); err != nil {
 		log.Fatalf("Unable to set up networking: %v", err)
 	}
 }