psa: Add storage implementation for files

Add new functions, psa_load_persistent_key(),
psa_free_persistent_key_data(), and psa_save_persistent_key(), for
managing persistent keys. These functions load to or save from our
internal representation of key slots. Serialization is a concern of the
storage backend implementation and doesn't abstraction-leak into the
lifetime management code.

An initial implementation for files is provided. Additional storage
backends can implement this interface for other storage types.

include/mbedtls/check_config.h

@@ -506,6 +506,18 @@
 #error "MBEDTLS_PSA_CRYPTO_SPM defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_PSA_CRYPTO_STORAGE_C) &&            \
+    !( defined(MBEDTLS_PSA_CRYPTO_C) &&           \
+       defined(MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C) )
+#error "MBEDTLS_PSA_CRYPTO_STORAGE_C defined, but not all prerequisites"
+#endif
+
+#if defined(MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C) &&            \
+    !( defined(MBEDTLS_PSA_CRYPTO_STORAGE_C) &&           \
+       defined(MBEDTLS_FS_IO) )
+#error "MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C defined, but not all prerequisites"
+#endif
+
 #if defined(MBEDTLS_RSA_C) && ( !defined(MBEDTLS_BIGNUM_C) ||         \
     !defined(MBEDTLS_OID_C) )
 #error "MBEDTLS_RSA_C defined, but not all prerequisites"

include/mbedtls/config.h

@@ -2616,6 +2616,31 @@
  */
 #define MBEDTLS_PSA_CRYPTO_C
 
+/**
+ * \def MBEDTLS_PSA_CRYPTO_STORAGE_C
+ *
+ * Enable the Platform Security Architecture persistent key storage.
+ *
+ * Module:  library/psa_crypto_storage.c
+ *
+ * Requires: MBEDTLS_PSA_CRYPTO_C, MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C
+ *
+ */
+#define MBEDTLS_PSA_CRYPTO_STORAGE_C
+
+/**
+ * \def MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C
+ *
+ * Enable persistent key storage over files for the
+ * Platform Security Architecture cryptography API.
+ *
+ * Module:  library/psa_crypto_storage_file.c
+ *
+ * Requires: MBEDTLS_PSA_CRYPTO_C, MBEDTLS_FS_IO
+ *
+ */
+#define MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C
+
 /**
  * \def MBEDTLS_RIPEMD160_C
  *