doc: define html_context in conf.py

The dictionary html_context is not passed into conf.py but must be created
there. See
https://dev.readthedocs.io/en/latest/design/theme-context.html#customizing-the-context

Fixes: df86796028df ("doc: enable ReadTheDocs addon management")
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/conf.py

@@ -26,7 +26,9 @@ html_baseurl = os.environ.get("READTHEDOCS_CANONICAL_URL", "")
 
 # Tell Jinja2 templates the build is running on Read the Docs
 if os.environ.get("READTHEDOCS", "") == "True":
-    html_context["READTHEDOCS"] = True
+    html_context = {
+        'READTHEDOCS' : True,
+    }
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the