Review: HelloVerifyRequest parsing

[hanno-becker]

This issue it to track the review and potential rework of the client-side HelloVerifyRequest state in preparation for upstreaming.

Things to look for:

• Security
• Functional correctness
• Clarity of code and information flow
• Adherence to Mbed TLS style

Acceptance criteria: One or more PRs improving on the above points to the satisfaction of the team, or a confirmation of review with the result that no changes were necessary.

[hanno-becker]

#102 by @gbryant-arm should be reviewed and merged prior to this.