Add check for return code in DeviceKey to avoid injecting invalid ROT

[boomer41]

Summary of changes

Check return code of the random generator before injecting the root of trust.
Without this check, an invalid - and therefore insecure - ROT may be used like in #9278.
This bug was introduced in #12385.

Impact of changes

Invalid ROTs won't be used.

Migration actions required

None. This bug was introduced in the v6-alpha and therefore isn't critical as the alpha may not be used for production devices.

Documentation

None. This bug was introduced in the v6-alpha and therefore isn't critical as the alpha may not be used for production devices.

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

[ciarmcom]

@boomer41, thank you for your changes.
@ARMmbed/mbed-os-storage @ARMmbed/mbed-os-maintainers please review.

[michalpasztamobica]

Looks OK to me.

[VeijoPesonen]

Looks good to me

[VeijoPesonen]

@teetak01 , @JanneKiiskila , @JammuKekkonen for your information.

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 1 of 7 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_greentea-test