initrd: reproducible build

[simone-weiss]

Changes

Set timestamp to SOURCE_DATE_EPOCH for initrd cpio content. When one exports SOURCE_DATE_EPOCH before calling the initrd generator twice for the same initrd, the built image is binary identical.

Dependencies:

(Provide list of pull requests with dependencies with this pull request)

Tests results

WIP need to implement reproducibility test.

<!-- add Robot test CLI logs here. -->

Developer Checklist:

• Test: Changes are tested
• Doc: Documentation has been updated
• Git: Informative git commit message(s)
• Issue: If a related GitHub issue exists, linking is done

Checklists for documentation

• Grammar: the content is grammatically correct
  (spelling, grammar, formatting, US English is used)
• Comprehensibility/Unambiguousness: the content is easy readable, clear to understand
• Correctness and consistency: the content is technically correct and consistent,
  no contradictions, no double descriptions
• Terminology: technical terms are clear and they are used correctly and documented in the glossary
• Level of detail: the content matches the detail level necessary for the reviewed object

Reviewer checklist:

• Review: Changes are reviewed
• Review: Tested by the reviewer

Checklists for documentation

• Grammar: the content is grammatically correct
  (spelling, grammar, formatting, US English is used)
• Comprehensibility/Unambiguousness: the content is easy readable, clear to understand
• Correctness and consistency: the content is technically correct and consistent,
  no contradictions, no double descriptions
• Terminology: technical terms are clear and they are used correctly and documented in the glossary
• Level of detail: the content matches the detail level necessary for the reviewed object