ref-manual, mega-manual: Added manifest file information

Fixes [YOCTO #5410]

I updated the figure that shows what do_rootfs does to create
the image.  The new addition is the creation of the .manifest file
that lists out package information in the image.  Supporting text
was also added in the section.

The updated figure was needed in both the ref-manual and
mega-manual figures folders.

Also, part of the fix included a new variable description for the
IMAGE_MANIFEST variable.

(From yocto-docs rev: 770448e0e8ce2e2c517ced5e71f5a916fefe5cf2)

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

documentation/ref-manual/closer-look.xml

@@ -970,6 +970,21 @@
                 executables.
             </para>
 
+            <para>
+                Along with writing out the root filesystem image, the
+                <filename>do_rootfs</filename> task creates a manifest file
+                (<filename>.manifest</filename>) in the same directory as
+                the root filesystem image that lists out, line-by-line, the
+                installed packages.
+                This manifest file is useful for the
+                <link linkend='ref-classes-testimage'><filename>testimage</filename></link>
+                class, for example, to determine whether or not to run
+                specific tests.
+                See the
+                <link linkend='var-IMAGE_MANIFEST'><filename>IMAGE_MANIFEST</filename></link>
+                variable for additional information.
+            </para>
+
             <para>
                 Part of the image generation process includes compressing the
                 root filesystem image.