bitbake: usermanual.xml: Add section for ?= var setting.

(Bitbake rev: df8309c11a43a9488ca25ec114a2dd3d49b041ff)

Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>

diff --git a/bitbake/doc/manual/usermanual.xml b/bitbake/doc/manual/usermanual.xml
index cdd05998a5a39cfdc70f8b71de0a9cbcf46ebc78..c3403449fe10797060e820b3ecabd914a0f3bd80 100644 (file)
--- a/bitbake/doc/manual/usermanual.xml
+++ b/bitbake/doc/manual/usermanual.xml
@@ -88,6 +88,11 @@ share common metadata between many packages.</para></listitem>
 <varname>B</varname> = "pre${A}post"</screen></para>
                 <para>This results in <varname>A</varname> containing <literal>aval</literal> and <varname>B</varname> containing <literal>preavalpost</literal>.</para>
             </section>
+            <section>
+                <title>Setting a default value (?=)</title>
+                <para><screen><varname>A</varname> ?= "aval"</screen></para>
+                <para>If <varname>A</varname> is set before the above is called, it will retain it's previous value. If <varname>A</varname> is unset prior to the above call, <varname>A</varname> will be set to <literal>aval</literal>.</para>
+            </section>
             <section>
                 <title>Immediate variable expansion (:=)</title>
                 <para>:= results in a variable's contents being expanded immediately, rather than when the variable is actually used.</para>