Darryl Green d49a499d03 psa: Implement persistent keys
Allow use of persistent keys, including configuring them, importing and
exporting them, and destroying them.

When getting a slot using psa_get_key_slot, there are 3 scenarios that
can occur if the keys lifetime is persistent:

1. Key type is PSA_KEY_TYPE_NONE, no persistent storage entry:
   -  The key slot is treated as a standard empty key slot
2. Key type is PSA_KEY_TYPE_NONE, persistent storage entry exists:
   -  Attempt to load the key from persistent storage
3. Key type is not PSA_KEY_TYPE_NONE:
   -  As checking persistent storage on every use of the key could
      be expensive, the persistent key is assumed to be saved in
      persistent storage, the in-memory key is continued to be used.
2018-11-20 15:40:25 +00:00
..
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