Introduce mbedtls_nv_seed_poll() entropy polling function

include/mbedtls/entropy_poll.h

@@ -3,7 +3,7 @@
  *
  * \brief Platform-specific and custom entropy polling functions
  *
- *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
+ *  Copyright (C) 2006-2016, ARM Limited, All Rights Reserved
  *  SPDX-License-Identifier: Apache-2.0
  *
  *  Licensed under the Apache License, Version 2.0 (the "License"); you may
@@ -90,6 +90,16 @@ int mbedtls_hardware_poll( void *data,
                            unsigned char *output, size_t len, size_t *olen );
 #endif
 
+#if defined(MBEDTLS_ENTROPY_NV_SEED)
+/**
+ * \brief           Entropy poll callback for a non-volatile seed file
+ *
+ * \note            This must accept NULL as its first argument.
+ */
+int mbedtls_nv_seed_poll( void *data,
+                          unsigned char *output, size_t len, size_t *olen );
+#endif
+
 #ifdef __cplusplus
 }
 #endif