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.

crypto/library/Makefile

@@ -45,6 +45,8 @@ OBJS_CRYPTO := \
 	platform.o \
 	platform_util.o \
 	psa_crypto.o \
+	psa_crypto_storage.o \
+	psa_crypto_storage_file.o \
 	ripemd160.o \
 	rsa_internal.o \
 	rsa.o \