Add mbedtls_x509_dn_get_next function

Allow iteration through relative DNs when X509 name contains multi-
value RDNs.

Signed-off-by: Werner Lewis <werner.lewis@arm.com>
diff --git a/include/mbedtls/x509.h b/include/mbedtls/x509.h
index 3c76fec..2ad90de 100644
--- a/include/mbedtls/x509.h
+++ b/include/mbedtls/x509.h
@@ -267,6 +267,16 @@
 int mbedtls_x509_dn_gets( char *buf, size_t size, const mbedtls_x509_name *dn );
 
 /**
+ * \brief          Return the next relative DN in an X509 name.
+ *
+ * \param dn       Current node in the X509 name
+ *
+ * \return         Pointer to the first attribute-value pair of the
+ *                 next RDN in sequence, or NULL if end is reached.
+ */
+mbedtls_x509_name * mbedtls_x509_dn_get_next( mbedtls_x509_name * start );
+
+/**
  * \brief          Store the certificate serial in printable form into buf;
  *                 no more than size characters will be written.
  *