openharmony-linux-demo-dashboard.conf: Add backwards compatible distro conf

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

meta-ohos-blueprints/conf/distro/openharmony-linux-demo-dashboard.conf

+# SPDX-FileCopyrightText: Huawei Inc.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+# Deprecated distro configuration in favour of
+# openharmony-linux-blueprint-dashboard
+
+require conf/distro/openharmony-linux-blueprint-dashboard.conf
+
+# We keep the old distro name to sanity check this deprecation.
+DISTRO = "openharmony-linux-demo-dashboard"