documentation: Prepared files for a 2.4.2 point release

poky.ent - Updated variables to reflect 2.4.2
mega-manual.sed - changes "2.4.1" string to "2.4.2"
<manual>.xml - Updated release manual table to use "TBD 2018"
               release date.

(From yocto-docs rev: cee187153d4136a90556fefbdaaf41bdc9d65006)

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

documentation/kernel-dev/kernel-dev.xml

@@ -91,6 +91,11 @@
                 <date>January 2018</date>
                 <revremark>Released with the Yocto Project 2.4.1 Release.</revremark>
             </revision>
+            <revision>
+                <revnumber>2.4.2</revnumber>
+                <date>TBD 2018</date>
+                <revremark>Released with the Yocto Project 2.4.2 Release.</revremark>
+            </revision>
         </revhistory>
 
     <copyright>