documentation: Added the memory-resident BitBake feature

The main changes are captured in a new section devoted to the
oe-init-build-env-memres script within the ref-structure.xml
file and in the variables.xml file in a BBSERVER variable entry
in the glossary.

All other changes were necessary to integrate the new functionality
into areas where running the setup script are discussed.  Before
this feature, there was a single setup script (oe-init-build-env).
Consequently, wordings and such were designed exclusively for this
single method.  With the introduction of a second possible method
to initialize the build environment, tweaks to many sections were
needed.  Beside the wording tweaks, appropriate cross-referencing
was also needed.

All these changes cover the integration of the new feature.

Reported-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(From yocto-docs rev: c55c9a53c9b4cd1e06e6e81e32d56f9bbf5fe98c)

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

documentation/ref-manual/usingpoky.xml

@@ -27,8 +27,13 @@
         <title>Build Overview</title>
 
         <para>
-            The first thing you need to do is set up the OpenEmbedded build environment by sourcing
-            the <link linkend='structure-core-script'>environment setup script</link> as follows:
+            The first thing you need to do is set up the OpenEmbedded build 
+            environment by sourcing an environment setup script
+            (i.e.
+            <link linkend='structure-core-script'><filename>&OE_INIT_FILE;</filename></link>
+            or
+            <link linkend='structure-memres-core-script'><filename>oe-init-build-env-memres</filename></link>).
+            Here is an example:
             <literallayout class='monospaced'>
      $ source &OE_INIT_FILE; [&lt;build_dir&gt;]
             </literallayout>