[Issue 546] Enable boot.d when running systemd init.

This change separates the overlays for sysvinit and systemd builds.

- The inittab is overlayed if using sysvinit.
- /sbin/init is replaced by a script which calls the boot.d sequence and then execs /lib/systemd/systemd when using systemd.

builds/any/rootfs/stretch/standard/standard.yml

@@ -66,6 +66,7 @@ Multistrap:
 Configure:
   overlays:
     - ${ONL}/builds/any/rootfs/${ONL_DEBIAN_SUITE}/common/overlay
+    - ${ONL}/builds/any/rootfs/${ONL_DEBIAN_SUITE}/${INIT}/overlay
 
   update-rc.d:
     - 'faultd defaults'

builds/any/rootfs/stretch/systemd/overlay/sbin/init

@@ -0,0 +1,4 @@
+#!/bin/bash
+/etc/boot.d/boot
+echo "Starting systemd..."
+exec /lib/systemd/systemd