doc: update Kernel documentation build system

Update the docomentation build system according to Linux v5.11-rc1.

With this patch we can build the HTML documentation using either of
Sphinx 2 and Sphinx 3.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
[Fix warning in b4860qds.rst about "Switch Settings"]
Signed-off-by: Tom Rini <trini@konsulko.com>

doc/sphinx/kernellog.py

@@ -25,4 +25,8 @@ def verbose(app, message):
     else:
         app.verbose(message)
 
-
+def info(app, message):
+    if UseLogging:
+        logger.info(message)
+    else:
+        app.info(message)