resin-init: flasher: add interface to purge encryption keys

[alexgg]

When re-flashing, make sure old encryption secrets are not available on the new system so they are not exploitable.

Change-type: patch

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Contributor checklist

• [ ] Changes have been tested
  • [ ] Covered in automated test suite
  • [ ] Manual test case recorded
• [ ] Change-type present on at least one commit
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[alexgg]

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