PSA_MAX_PERSISTENT_KEY_IDENTIFIER macro

The maximum permitted persistent slot number. In Mbed Crypto 0.1.0b: - Using the file backend, all key ids are ok except 0. - Using the ITS backend, all key ids are ok except 0xFFFFFF52 (#PSA_CRYPTO_ITS_RANDOM_SEED_UID) for which the file contains the device's random seed (if this feature is enabled). - Only key ids from 1 to #MBEDTLS_PSA_KEY_SLOT_COUNT are actually used. Since we need to preserve the random seed, avoid using that key slot. Reserve a whole range of key slots just in case something else comes up. This limitation will probably become moot when we implement client separation for key storage.