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/CMakeLists.txt b/library/CMakeLists.txt
index 6a280fe..07811f9 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -53,6 +53,7 @@
     platform.c
     platform_util.c
     poly1305.c
+    psa_crypto.c
     ripemd160.c
     rsa.c
     rsa_internal.c