ref-manual: document IMAGE_BOOT_FILES

Document IMAGE_BOOT_FILES variable.

(From yocto-docs rev: f0b5cbcde4b83911780ab525f279dcb527fb1839)

Signed-off-by: Maciej Borzecki <maciej.borzecki@open-rnd.pl>
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-variables.xml

@@ -3257,6 +3257,24 @@
             </glossdef>
         </glossentry>
 
+        <glossentry id='var-IMAGE_BOOT_FILES'><glossterm>IMAGE_BOOT_FILES</glossterm>
+            <glossdef>
+                <para>
+                    Comma separated list of files that will be installed into
+                    boot partition when preparing an image. Source files By default the file
+                    is installed under the same name as source file. To change
+                    the destination name, separate it with &quot;;&quot;. Source files
+                    need to be located in
+                    <link linkend='var-DEPLOY_DIR_IMAGE'><filename>DEPLOY_DIR_IMAGE</filename></link>.
+                    Examples:
+
+                    <literallayout class="monospaced">
+     IMAGE_BOOT_FILES = "u-boot.img uImage:kernel"
+     IMAGE_BOOT_FILES = "u-boot.${UBOOT_SUFFIX} ${KERNEL_IMAGETYPE}""
+                    </literallayout></para>
+            </glossdef>
+        </glossentry>
+
         <glossentry id='var-IMAGE_CLASSES'><glossterm>IMAGE_CLASSES</glossterm>
             <glossdef>
                 <para>