move verity generation to imghelper

[jpculp]

Description of changes:

Moves verity code-block to imghelper.

Testing done:

• Built and smoke tested aws-k8s-1.28.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[jpculp]

Switched from generating verity values to the dm_verity_root directly.

[jpculp]

• Redeclared some vars as local.
• Adjusted formatting.
• Noticed a missing redirect to stderr.

[jpculp]

Rebased