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.
diff --git a/scripts/mbed_crypto.make b/scripts/mbed_crypto.make
index ab54d55..f51f5f8 100644
--- a/scripts/mbed_crypto.make
+++ b/scripts/mbed_crypto.make
@@ -158,6 +158,8 @@
 	tests/suites/test_suite_psa_crypto_hash.function \
 	tests/suites/test_suite_psa_crypto_metadata.data \
 	tests/suites/test_suite_psa_crypto_metadata.function \
+	tests/suites/test_suite_psa_crypto_persistent_key.data \
+	tests/suites/test_suite_psa_crypto_persistent_key.function \
 	tests/suites/test_suite_psa_crypto_storage_file.data \
 	tests/suites/test_suite_psa_crypto_storage_file.function \
 # Don't delete this line.