ref-manual: add DEFAULT_TIMEZONE variable

(From yocto-docs rev: 89323e4d95c7e98c685ad2843f037a291af08b02)

Signed-off-by: Jörg Sommer <joerg.sommer@navimatix.de>
Reviewed-by: Michael Opdenacker <michael@opdenacker.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -1877,6 +1877,13 @@ system and gives an overview of their function and contents.
          by :term:`BBFILE_PRIORITY` if that variable is different between two
          layers that contain different versions of the same recipe.
 
+   :term:`DEFAULT_TIMEZONE`
+      Specifies the time zone set in the image.
+
+      This variable causes the ``tzdata`` package to configure
+      ``${sysconfdir}/localtime`` accordingly. Valid values are all files
+      found in ``/usr/share/zoneinfo`` like ``CET`` or ``Asia/Baku``.
+
    :term:`DEFAULTTUNE`
       The default CPU and Application Binary Interface (ABI) tunings (i.e.
       the "tune") used by the OpenEmbedded build system. The