Add support for the legacy_boot flag on GPT to manual partitioning

On GPT disk label, the "boot" flag actually means "esp", so sync the "B" visual marker with the "legacy_boot" flag instead. This flag is needed to boot on some arm64 platorms.

partman-partitioning!5 (merged) is also required for full support.

changed the description

mentioned in merge request partman-partitioning!5 (merged)

I couldn't see any issues in tests, so I think it is ok to merge this, together with partman-partitioning!5 (merged)

merged

mentioned in commit bc69e1fa