oniro-linux.conf: Define a default OpenHarmony version

Without this, building with distro feature `openharmony` fails in
various ways:

- parsing
- applying patches
- unpacking sources
- etc.

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

meta-oniro-core/conf/distro/oniro-linux.conf

@@ -115,3 +115,5 @@ SPLASH = "psplash-oniro"
 
 DISTRO_FEATURES_FILTER_NATIVE:append = " openharmony"
 DISTRO_FEATURES_FILTER_NATIVESDK:append = " openharmony"
+OPENHARMONY_VERSION ?= "3.0"
+PREFERRED_VERSION_openharmony-standard = "${OPENHARMONY_VERSION}"