bootupd integration

Currently the `bootc upgrade/switch` commands do not install/add static grub configs (via bootupd). Note, that this feature was added to bootupd via https://github.com/coreos/bootupd/pull/543.

This is essential part for supporting [greenboot](https://github.com/fedora-iot/greenboot). Without installing grub static configs one has to have an image with already the grub configs installed or add them manually.

See also https://github.com/fedora-iot/greenboot/issues/135.

Thank you.

Start by tracing the implementations of the bootc upgrade and switch commands, then read bootupd pull request #543 to understand the static GRUB configuration support. Done means both commands install or add the required static GRUB configs, enabling the greenboot use case described in the issue.

Vom Indexierungsmodell aus dem Issue-Text verfasst.