Instead of hardcoding an `openharmony` override, this introduce an
`openharmony` distro feature which (when enabled) will add a
`df-openharmony` override.

The `openharmony` distro feature will also gate the `openharmony-3.0`
and `openharmony-3.1` overrides, so they are only active when `openharmony`
is enabled.

Signed-off-by: Esben Haabendal <esben.haabendal@huawei.com>