Add PSA crypto module

New module psa_crypto.c (MBEDTLS_PSA_CRYPTO_C):
Platform Security Architecture compatibility layer on top of
libmedcrypto.

Implement psa_crypto_init function which sets up a RNG.

Add a mbedtls_psa_crypto_free function which deinitializes the
library.

Define a first batch of error codes.
diff --git a/library/Makefile b/library/Makefile
index 430c598..f4b39bd 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -81,6 +81,7 @@
 		pk.o		pk_wrap.o	pkcs12.o	\
 		pkcs5.o		pkparse.o	pkwrite.o	\
 		platform.o	platform_util.o	poly1305.o	\
+		psa_crypto.o					\
 		ripemd160.o	rsa_internal.o	rsa.o  		\
 		sha1.o		sha256.o	sha512.o	\
 		threading.o	timing.o	version.o	\