Add a subcommand to generate stboot disk image

Created by: sylv-io

At the current state, the system-transparency repository uses scripts to create stboot installation as disk or ISO images. They require many build dependencies and are the only part of the build-process, which is not integrated into the tooling.

Golang provides all necessary libraries to implement a solid way to generate disk images. It would reduce the overall build dependencies to Golang and Linux coreutils and make the build environment less dependent on the Linux distribution. Additional Arguments:

• more control over binary reproducibility
• simpler SBOM support in future
• better integration test support (fewer regressions)
• Linux Distro independent (even macOS could be supported)