build images suitable for several block sizes

[wfeldt]

When partitioning images, you have to decide for a block size. But it is possible to have GPTs for different block sizes on the same disk. This allows to build images that work (for example) for 512 bytes and 4096 bytes block sizes at the same time.

See

• https://github.com/wfeldt/parti/blob/master/README_unify_gpt.md

[github-actions]

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