Kernel: T861: add custom VyOS CA to Kernel builds for later module signing

[c-po]

Change summary

With this addition we can always sign a Kernel module later and ship it if needed, without re-compiling the Kernel.

Kernel will report:
[ 1.223891] Loaded X.509 cert 'VyOS Networks Secure Boot Signer 2025 - linux: 6ca57e2add335babd08da69b48c70693edd2b037'

Issuer: CN = VyOS Networks Secure Boot CA
        Validity
            Not Before: Apr 26 09:07:06 2025 GMT
            Not After : Apr 24 09:07:06 2035 GMT
        Subject: CN = VyOS Networks Secure Boot Signer 2025 - linux

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):

Related Task(s)

• https://vyos.dev/T861

Related PR(s)

How to test / Smoketest result

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

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[github-actions]

👍
No issues in PR Title / Commit Title

[sever-sever]

Add CA cert for signing modules.

[github-actions]

CI integration 👍 passed!

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CI logs

• Config tests 👍 passed

[jestabro]

Add CA for signing of modules.