cypress: make the security counter stub fail closed

[flaviens]

Hi!

The example backend returned a fixed counter value and discarded updates while reporting success. It linked and ran with a rollback floor that never advanced with no error at build or boot time because a build that enables MCUBOOT_HW_ROLLBACK_PROT picks this file up automatically.

This PR proposes to return errors instead and document that a persistent platform backend is required before enabling MCUBOOT_HW_ROLLBACK_PROT.

Thanks!
Flavien

[Copilot]

Pull request overview

This PR updates the Cypress example security counter backend to “fail closed” when MCUBOOT_HW_ROLLBACK_PROT is enabled, preventing builds from silently operating with a non-advancing rollback floor due to a non-persistent stub implementation.

Changes:

• Make the Cypress cy_security_cnt.c stub return failures instead of returning a fixed counter value and reporting success.
• Document in the Cypress MCUboot app README that a persistent, monotonic, power-fail-safe backend is required before enabling MCUBOOT_HW_ROLLBACK_PROT.
• Add a release note entry describing the behavioral change.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
docs/release-notes.d/cypress-security-counter-fail-closed.md	Adds a release note explaining the Cypress security counter backend now fails closed.
boot/cypress/MCUBootApp/README.md	Documents that the bundled security counter backend is non-persistent and must be replaced before enabling rollback protection.
boot/cypress/MCUBootApp/cy_security_cnt.c	Changes the example backend to return errors (fail closed) rather than returning a fixed value and succeeding.

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