adt-manual, ref-manual: Detail on using an external toolchain.

Fixes [YOCTO #4907]

I created a new section in the toolchain chapter of the adt-manual
called "optionally Using an External Toolchain".  This provides
some fundamental information for the user that wants to use
an external toolchain.  Additionally, it references Mentor
Graphics Sourcery toolchain as an example.

In the ref-manual, I updated the TCMODE variable description to
better integrate the MGC Sourcery toolchain as the overriding
example.

(From yocto-docs rev: 51e9c3b88f7e5a16da2aa101589b0f2612fff11d)

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

documentation/adt-manual/adt-prepare.xml

@@ -709,6 +709,45 @@
     </para>
 </section>
 
+<section id='optionally-using-an-external-toolchain'>
+    <title>Optionally Using an External Toolchain</title>
+
+    <para>
+        You might want to use an external toolchain as part of your
+        development.
+        If this is the case, the fundamental steps you need to accomplish
+        are as follows:
+        <itemizedlist>
+            <listitem><para>
+                Understand where the installed toolchain resides.
+                For cases where you need to build the external toolchain, you
+                would need to take separate steps to build and install the
+                toolchain.
+                </para></listitem>
+            <listitem><para>
+                Make sure you add the layer that contains the toolchain to
+                your <filename>bblayers.conf</filename> file through the
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-BBLAYERS'><filename>BBLAYERS</filename></ulink>
+                variable.
+                </para></listitem>
+            <listitem><para>
+                Set the
+                <ulink url='&YOCTO_DOCS_REF_URL;#var-EXTERNAL_TOOLCHAIN'><filename>EXTERNAL_TOOLCHAIN</filename></ulink>
+                variable in your <filename>local.conf</filename> file
+                to the location in which you installed the toolchain.
+                </para></listitem>
+        </itemizedlist>
+        A good example of an external toolchain used with the Yocto Project
+        is <trademark class='registered'>Mentor Graphics</trademark>
+        Sourcery G++ Toolchain.
+        You can see information on how to use that particular layer in the
+        <filename>README</filename> file at
+        <ulink url='http://github.com/MentorEmbedded/meta-sourcery/'></ulink>.
+        You can find further information by reading about the
+        <ulink url='&YOCTO_DOCS_REF_URL;#var-TCMODE'><filename>TCMODE</filename></ulink>
+        variable in the Yocto Project Reference Manual's variable glossary.
+    </para>
+</section>
 </chapter>
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