Allow upgrades from v2020 to v2021

• Produce ostree commit which allow to upgrade Apertis v2020 to v2021 versions family starting from v2021pre
• Explicitly generate the additional bundle with unsigned metadata to allow offline upgrades for previous release v2020. Need that since the delta file with signed metadata is not compatible with all Apertis releases using libostree version prior v2020.7, i.e. apertis/v2020.