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.
diff --git a/library/Makefile b/library/Makefile
index cf6750d..83afa66 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -82,6 +82,8 @@
 		pkcs5.o		pkparse.o	pkwrite.o	\
 		platform.o	platform_util.o	poly1305.o	\
 		psa_crypto.o					\
+		psa_crypto_storage.o				\
+		psa_crypto_storage_file.o			\
 		ripemd160.o	rsa_internal.o	rsa.o  		\
 		sha1.o		sha256.o	sha512.o	\
 		threading.o	timing.o	version.o	\