ECDSA related PRs

[Bob Wolff]

Hi there,
I have two separate but related pull requests I'd like to contribute. They
both have to do with ECDSA support.
- The simple one is a lack of null-pointer check that can cause a crash in
certain situations. Easy peasy.
- The less simple one (and hopefully not too controversial) adds an ecdsa
verify driver (UCLASS_ECDSA) which utilizes tinycrypt to do the crypto work.

Please advise on how best to proceed. Happy to work within the confines of
what works best for the larger group.

Thanks,
Bob Wolff