ref-manual: Added BUILDHISTORY_SDK variable.

(From yocto-docs rev: 41ee162c260c1c9b6b691f9052af53ff469b73df)

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

@@ -965,6 +965,30 @@
             </glossdef>
         </glossentry>
 
+        <glossentry id='var-BUILDHISTORY_DIR_SDK'><glossterm>BUILDHISTORY_DIR_SDK</glossterm>
+            <glossdef>
+                <para>
+                    When inheriting the
+                    <link linkend='ref-classes-buildhistory'><filename>buildhistory</filename></link>
+                    class, specifies the name of the software development kit
+                    (SDK) analyzed using the build history feature.
+                    For more information on how the variable works, see the
+                    <filename>buildhistory.class</filename>.
+                    For more information on how build history works, see the
+                    "<link linkend='maintaining-build-output-quality'>Maintaining Build Output Quality</link>"
+                    section.
+                </para>
+
+                <para>
+                    By default, the <filename>buildhistory</filename> class
+                    sets the SDK name as follows:
+                    <literallayout class='monospaced'>
+     BUILDHISTORY_DIR_SDK = "${BUILDHISTORY_DIR}/sdk/${SDK_NAME}/${IMAGE_BASENAME}"
+                    </literallayout>
+                </para>
+            </glossdef>
+        </glossentry>
+
         <glossentry id='var-BUILDHISTORY_FEATURES'><glossterm>BUILDHISTORY_FEATURES</glossterm>
             <glossdef>
                 <para>