documentation: Prepared 2.3.1 release variables and dates

poky.ent - updated variables for 2.3.1
mega-manual.sed - exchanged "2.3.1" for "2.3" for links
<manual>.xml - updated manual revision tables to use
               "June 2017" as the release date for 2.3.1

(From yocto-docs rev: 01d60d08a0c7371b8f7476f45fca89226caec680)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/kernel-dev/kernel-dev.xml

@@ -81,6 +81,11 @@
                 <date>May 2017</date>
                 <revremark>Released with the Yocto Project 2.3 Release.</revremark>
             </revision>
+            <revision>
+                <revnumber>2.3.1</revnumber>
+                <date>June 2017</date>
+                <revremark>Released with the Yocto Project 2.3.1 Release.</revremark>
+            </revision>
         </revhistory>
 
     <copyright>