samples: tfm_integration: Extend PSA cryptography demo

This commit extends the psa_level_1 sample's use of the PSA
Cryptography 1.0 API to demonstrate the following functionality:

- Generate a persistent key (secp256r1)
- Display the public key based on the private key above
- Calculates the SHA256 hash of a payload
- Signs the hash with the persistent key
- Verifies the signature using the public key
- Destroys the key

Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>