Implement the openless crypto API

[gilles-peskine-arm]

Mbed Crypto currently implements version 1.0 beta 3 of the PSA Crypto API. In the final version of the API, there has been one major change to key management. Now, instead of calling psa_open_key on a persistent key identifier to obtain a transient handle to the key, the application directly uses the identifier as a handle. This is not yet possible in Mbed Crypto.

The goal of this task is to implement the openless API:

• Use psa_key_id_t wherever the API currently has psa_key_handle_t.
• Since psa_key_id_t is not restricted to a small range, there needs to be more indirection in the equivalent function to psa_get_key_slot for key ids. Possible methods include:
  • A linear search in the existing table global_data.key_slots.
  • Another table mapping key ids to indices in global_data.key_slots.
• Implement the new function psa_purge_key.
• Add tests that exercise the new behavior. It's ok to keep calls to the old functions in most existing tests.

The behavior if a key is purged while a multipart operation is in progress is out of scope of this task and tracked separately at https://github.com/ARMmbed/mbed-crypto/issues/86. However, there are commonalities in the design, so a prerequisite of this task is to come up with a design that's compatible with both problems.

For backward compatibility, keep the following declared in include/psa/crypto_compat.h:

• psa_open_key either as a function that only checks whether the key exists, or essentially as today (loading the key into memory).
• psa_close_key at least calls psa_purge_key.
• psa_key_handle_t is an alias of psa_key_id_t.

[ronald-cron-arm]

#3527 merged but not #3547 thus reopening this issue that has been closed automatically due to the merge of #3527.