ref-manual: add usage NOTE to IMAGE_FSTYPES

Due to the way in which IMAGE_FSTYPES is processed, it is not possible to
modify it using _append or _prepend. Therefore add a note to the manual to
warn users in case they stumble on this issue.

(From yocto-docs rev: 304e196329842d04356775fb8ad5d73466413e66)

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

1 files changed, 9 insertions, 0 deletions

diff --git a/documentation/ref-manual/ref-variables.xml b/documentation/ref-manual/ref-variables.xml
index fb337913ca..67ea422618 100644
--- a/documentation/ref-manual/ref-variables.xml
+++ b/documentation/ref-manual/ref-variables.xml
@@ -2199,6 +2199,15 @@
                     you can choose, see
                     <link linkend='var-IMAGE_TYPES'><filename>IMAGE_TYPES</filename></link>.
                 </para>
+
+                <note>
+                    Due to the way this variable is processed, it is not
+                    possible to update its contents using
+                    <filename>_append</filename> or
+                    <filename>_prepend</filename>. To add one or more
+                    additional options to this variable the
+                    <filename>+=</filename> operator must be used.
+                </note>
             </glossdef>
         </glossentry>