qtbase-native: use QT_CONFIG_FLAGS variable like qtbase and nativesdk-qtbase
qtbase-native: use QT_CONFIG_FLAGS variable like qtbase and nativesdk-qtbase

* instead of using PACKAGECONFIG_CONFARGS directly

Signed-off-by: Martin Jansa <516df3ff67e1fa5d153800b15fb204c0a1a389dc@gmail.com>

meta-qt5/meta-qt5,sapiippo/meta-qt5,schnitzeltony/meta-qt5,sapiippo/meta-qt5,meta-qt5/meta-qt5,schnitzeltony/meta-qt5