ref-manual: Added IMAGE_CMD variable.

(From yocto-docs rev: 2050839776033fcd25ed4f9f376b9f2c44c3d235)

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

@@ -3148,6 +3148,33 @@
             </glossdef>
         </glossentry>
 
+        <glossentry id='var-IMAGE_CMD'><glossterm>IMAGE_CMD</glossterm>
+            <glossdef>
+                <para>
+                    Specifies the command to create the image file for a
+                    specific image type, which corresponds to the value set
+                    set in
+                    <link linkend='var-IMAGE_FSTYPES'><filename>IMAGE_FSTYPES</filename></link>,
+                    (e.g. <filename>ext3</filename>,
+                    <filename>btrfs</filename>, and so forth).
+                    When setting this variable, you should use
+                    an override for the associated type.
+                    Here is an example:
+                    <literallayout class='monospaced'>
+     IMAGE_CMD_jffs2 = "mkfs.jffs2 --root=${IMAGE_ROOTFS} \
+        --faketime --output=${DEPLOY_DIR_IMAGE}/${IMAGE_NAME}.rootfs.jffs2 \
+        ${EXTRA_IMAGECMD}"
+                    </literallayout>
+                    You typically do not need to set this variable unless
+                    you are adding support for a new image type.
+                    For more examples on how to set this variable, see the
+                    <link linkend='ref-classes-image_types'><filename>image_types</filename></link>
+                    class file, which is
+                    <filename>meta/classes/image_types.bbclass</filename>.
+                </para>
+            </glossdef>
+        </glossentry>
+
         <glossentry id='var-IMAGE_DEVICE_TABLES'><glossterm>IMAGE_DEVICE_TABLES</glossterm>
             <glossdef>
                 <para>