build: T7409: do not include MD5 checksums in the ISO image

[dmbaturin]

Change summary

It simply removes MD5 sums from the internal image integrity check and leaves only SHA-256.

I verified that it has no effect on upgrade (even from 1.3.8) or on live CD boot.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe): internal change.

Related Task(s)

Related PR(s)

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

👍
No issues in PR Title / Commit Title

[c-po]

Change looks reasonable. Older supported images ship md5 as interim step to enable sha256 verification in upcoming images.

Local build passes, too