Refactor the things that are common between bird and frr build types -- no-op for the build/ output

overlays/_common/tap/etc/rc.local

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+ip link set enp16s0f0 alias mirror-rx mtu 9216 up
+ip link set enp16s0f1 alias mirror-tx mtu 9216 up

overlays/_common/tap/root/lab-build/disable-vpp

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+echo " * Disabling VPP, Bird and FRR"
+for i in vpp vppcfg bird-dataplane frr; do
+  rm -f /etc/systemd/system/multi-user.target.wants/${i}.service
+  ln -sf /dev/null /etc/systemd/system/${i}.service
+done