app: use extra_overlay_confs

Using OVERLAY_CONFIG in extra_args is deprecated in favor of
extra_overlay_confs.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

Author: gmarull

app/sample.yaml

@@ -12,4 +12,5 @@ common:
 tests:
   app.default: {}
   app.debug:
-    extra_args: OVERLAY_CONFIG="debug.conf"
+    extra_overlay_confs:
+      - debug.conf