ECDSA: Refactor mbedtls_ecdsa_signature_to_raw

Change mbedtls_ecdsa_signature_to_raw so that it does not use MPI.
Add documentation changes.
diff --git a/include/mbedtls/ecdsa.h b/include/mbedtls/ecdsa.h
index bff30fc..2b25aa6 100644
--- a/include/mbedtls/ecdsa.h
+++ b/include/mbedtls/ecdsa.h
@@ -256,8 +256,8 @@
  */
 int mbedtls_ecdsa_signature_to_raw( const unsigned char *sig,
                             size_t ssize, uint16_t byte_len,
-                            unsigned char *buf, size_t bufsize,
-                            size_t* buflen );
+                            unsigned char *buf, size_t* buflen,
+                            size_t bufsize );
 /**
  * \brief           Convert a signature from numbers to ASN.1
  *