ref-manual: Added TUNE_ARCH variable and edited TARGET_ARCH.

Fixes [YOCTO 5463]

Added the TUNE_ARCH variable.  Also placed a cross-reference to
TUNE_ARCH in the TARGET_ARCH variable.

(From yocto-docs rev: 85d2a49323e0040b263b6d7f08fd036a25297b05)

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

documentation/ref-manual/ref-variables.xml

@@ -8200,6 +8200,10 @@ recipes-graphics/xorg-font/font-alias_1.0.3.bb:PR = "${INC_PR}.3"
      mips
      mipsel
                     </literallayout>
+                    For additional information on machine architectures, see
+                    the
+                    <link linkend='var-TUNE_ARCH'><filename>TUNE_ARCH</filename></link>
+                    variable.
                 </para>
             </glossdef>
         </glossentry>
@@ -8796,6 +8800,61 @@ recipes-graphics/xorg-font/font-alias_1.0.3.bb:PR = "${INC_PR}.3"
             </glossdef>
         </glossentry>
 
+        <glossentry id='var-TUNE_ARCH'><glossterm>TUNE_ARCH</glossterm>
+            <glossdef>
+                <para>
+                    The GNU canonical architecture for a specific architecture
+                    (i.e. <filename>arm</filename>,
+                    <filename>armeb</filename>,
+                    <filename>mips</filename>,
+                    <filename>mips64</filename>, and so forth).
+                    BitBake uses this value to setup configuration.
+                </para>
+
+                <para>
+                    <filename>TUNE_ARCH</filename> definitions are specific to
+                    a given architecture.
+                    The definitions can be a single static definition, or
+                    can be dynamically adjusted.
+                    You can see details for a given CPU family by looking at
+                    the architecture's <filename>README</filename> file.
+                    For example, the
+                    <filename>meta/conf/machine/include/mips/README</filename>
+                    file in the
+                    <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>
+                    provides information for <filename>TUNE_ARCH</filename>
+                    specific to the <filename>mips</filename> architecture.
+                </para>
+
+                <para>
+                    <filename>TUNE_ARCH</filename> is tied closely to
+                    <link linkend='var-TARGET_ARCH'><filename>TARGET_ARCH</filename></link>,
+                    which defines the target machine's architecture.
+                    The BitBake configuration file
+                    (<filename>meta/conf/bitbake.conf</filename>) sets
+                    <filename>TARGET_ARCH</filename> as follows:
+                    <literallayout class='monospaced'>
+     TARGET_ARCH = "${TUNE_ARCH}"
+                    </literallayout>
+                </para>
+
+                <para>
+                    The following list, which is by no means complete since
+                    architectures are configurable, shows supported machine
+                    architectures:
+                    <literallayout class='monospaced'>
+     arm
+     i586
+     x86_64
+     powerpc
+     powerpc64
+     mips
+     mipsel
+                    </literallayout>
+                </para>
+            </glossdef>
+        </glossentry>
+
         <glossentry id='var-TUNE_CCARGS'><glossterm>TUNE_CCARGS</glossterm>
             <glossdef>
                 <para>