dev-manual, ref-manual: Added notes for Systemd-based images.

Fixes [YOCTO #5159]

In the dev-manual, I located instances where a note about using
VIRTURAL-RUNTIME_initscripts = "" in the local.conf file would
be appropriate.  Three areas updated.

In the ref-manual, I located the note in the systemd.bbclass
section and the variable descriptions for SYSTEM_AUTO_ENABLE,
SYSTEM_PACKAGES, and SYSTEMD_SERVICE.

(From yocto-docs rev: 57b0acf2f3f8159b635b42dcffa129f7d945eb81)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/dev-manual-common-tasks.xml

@@ -2096,7 +2096,13 @@
                         See the <filename>systemd.class</filename> file
                         located in your
                         <link linkend='source-directory'>Source Directory</link>.
-                        section for more information.</para></listitem>
+                        section for more information.
+                        <note>For Systemd-based images, include the following
+                        in your <filename>local.conf</filename> file:
+                        <literallayout class='monospaced'>
+     VIRTUAL-RUNTIME_initscripts = ""
+                        </literallayout>
+                        </note></para></listitem>
                 </itemizedlist>
             </para>
         </section>
@@ -5355,6 +5361,14 @@
         <section id='using-systemd-exclusively'>
             <title>Using Systemd Exclusively</title>
 
+            <para>
+                Set this variable in your <filename>local.conf</filename>
+                file:
+                <literallayout class='monospaced'>
+     VIRTUAL-RUNTIME_initscripts = ""
+                </literallayout>
+            </para>
+
             <para>
                 Set the these variables in your distribution configuration
                 file as follows:
@@ -5380,6 +5394,14 @@
         <section id='using-systemd-for-the-main-image-and-using-sysvinit-for-the-rescue-image'>
             <title>Using Systemd for the Main Image and Using SysVinit for the Rescue Image</title>
 
+            <para>
+                Set this variable in your <filename>local.conf</filename>
+                file:
+                <literallayout class='monospaced'>
+     VIRTUAL-RUNTIME_initscripts = ""
+                </literallayout>
+            </para>
+
             <para>
                 Set the these variables in your distribution configuration
                 file as follows: