Add init-scripts directory support

There is a common pattern of if init_system == foo then install init
script foo-service-init into /etc/init... Lets encode this pattern by
allowing elements to put files into init-scripts/init-system directories
and then copying the appropriate files for them.

Change-Id: I541db18a0a8c5e0755a0af5732f4e15a5e5cf984

elements/dib-init-system/README.rst

@@ -2,4 +2,17 @@
 dib-init-system
 ===============
 
-A simple element that provides a script to tell what the init system is.
+Installs a script (dib-init-system) which outputs the type of init system in
+use on the target image. Also sets an environment variable ``DIB_INIT_SYSTEM``
+to this value.
+
+Any files placed in a ``init-scripts/INIT_SYSTEM`` directory inside the
+element will be copied into the appropriate directory if ``INIT_SYSTEM``
+is in use on the host.
+
+Environment Variables
+---------------------
+
+DIB_INIT_SYSTEM
+  :Description: One of upstart, systemd, or sysv depending on the init system
+    in use for the target image.

elements/dib-init-system/install.d/20-install-init-scripts

@@ -0,0 +1,26 @@
+#!/bin/bash
+# Note that this relies on the detail that all elements share one dir inside
+# the chroot. This will copy all the files that elements have added to
+# element/static into the image. Mode, symlinks etc will be respected.
+
+if [ ${DIB_DEBUG_TRACE:-1} -gt 0 ]; then
+    set -x
+fi
+set -eu
+set -o pipefail
+
+scripts_dir="$(dirname $0)../init-scripts/$DIB_INIT_SYSTEM/"
+if [ -d "$scripts_dir" ]; then
+    dest=
+    case $DIB_INIT_SYSTEM in
+        upstart) dest=/etc/init/ ;;
+        systemd) dest=/usr/lib/systemd/system/ ;;
+        sysv) dest=/etc/init.d/ ;;
+    esac
+
+    if [ -z "$dest" ]; then
+        echo "ERROR: DIB_INIT_SYSTEM ($DIB_INIT_SYSTEM) is not an unknown type"
+        exit 1
+    fi
+    rsync -lCr "$scripts_dir" $dest
+fi