bitbake: doc/bitbake-user-manual-ref-variables: document BB_DEFER_BBCLASSES

Document the BB_DEFER_BBCLASSES variable added in 8e741b2e885a
("ast/BBHandler: Add support for BB_DEFER_BBCLASSES").

(Bitbake rev: 3f9eba26b29db526fed263a7e70b824988e17656)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst

@@ -871,6 +871,9 @@ In all cases, if the expression evaluates to an
 empty string, the statement does not trigger a syntax error because it
 becomes a no-op.
 
+See also :term:`BB_DEFER_BBCLASSES` for automatically promoting classes
+``inherit`` calls to ``inherit_defer``.
+
 ``include`` Directive
 ---------------------
 

bitbake/doc/bitbake-user-manual/bitbake-user-manual-ref-variables.rst

@@ -141,6 +141,25 @@ overview of their function and contents.
       The default umask to apply to tasks if specified and no task specific
       umask flag is set.
 
+   :term:`BB_DEFER_BBCLASSES`
+      The classes listed in this variable have their :ref:`inherit
+      <ref-bitbake-user-manual-metadata-inherit>` calls automatically promoted
+      to deferred inherits. See :ref:`inherit_defer
+      <ref-bitbake-user-manual-metadata-inherit-defer>` for more information on
+      deferred inherits.
+
+      This means that if :term:`BB_DEFER_BBCLASSES` is set as follows::
+
+         BB_DEFER_BBCLASSES = "foo"
+
+      The following statement::
+
+         inherit foo
+
+      Will automatically be equal to calling::
+
+         inherit_defer foo
+
    :term:`BB_DISKMON_DIRS`
       Monitors disk space and available inodes during the build and allows
       you to control the build based on these parameters.