Remove internal file references in programs/

`entropy_poll.h` and `md_wrap.h` were still being used in some of the
example programs. As these headers are now internal, remove their
references and replace them with publicly available functions.

Signed-off-by: Chris Jones <christopher.jones@arm.com>

include/mbedtls/entropy.h

@@ -134,6 +134,14 @@ typedef struct mbedtls_entropy_context
 }
 mbedtls_entropy_context;
 
+#if !defined(MBEDTLS_NO_PLATFORM_ENTROPY)
+/**
+ * \brief           Platform-specific entropy poll callback
+ */
+int mbedtls_platform_entropy_poll( void *data,
+                           unsigned char *output, size_t len, size_t *olen );
+#endif
+
 /**
  * \brief           Initialize the context
  *