Key derivation: export secret key from secure element

[gilles-peskine-arm]

This a step towards the implementation of the interface for key derivation drivers introduced in #5451. The goal of this issue is to implement the simple case of key derivation with a secret in a secure element, by exporting the key from the secure element.

• Change psa_key_derivation_input_key to export the key, if it's in a secure element.
• Add a unit test with a key derivation where the SECRET input step is a key in a secure element. Note that the secure element driver does not need to have any key derivation entry point, only an export_key entry point.

(Deriving a key in a secure element without exporting it will be implemented later in #5486.)