dev-manual, ref-manual: Added support for WKS_FILE_DEPENDS variable.

Fixes [YOCTO #11017]

There is no current way to specify or list plug-in dependencies
when building a Wic image.  The variable WKS_FILE_DEPENDS was introduced
to allow the user to use this in the recipe building the image.
I added a description and example of the variable to the ref-manual
variable glossary.  I also cross-referenced the variable in the
"Plug-ins" section of the dev-manual to point to the variable for
more information.

(From yocto-docs rev: 5a1072499ab80f74ffec6af2cc0b2f1987843a25)

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

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

@@ -5038,6 +5038,14 @@
                     <filename>--source</filename> keyword to a
                     particular plug-in implementation that populates a
                     corresponding partition.
+                    <note>
+                        If you use plug-ins that have build-time dependencies
+                        (e.g. native tools, bootloaders, and so forth)
+                        when building a Wic image, you need to specify those
+                        dependencies using the
+                        <ulink url='&YOCTO_DOCS_REF_URL;#var-WKS_FILE_DEPENDS'><filename>WKS_FILE_DEPENDS</filename></ulink>
+                        variable.
+                    </note>
                 </para>
 
                 <para>