ECDSA related PRs

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