notls, tinydtls: Add stubs for unused TLS functions.

src/coap_notls.c

@@ -110,6 +110,14 @@ unsigned int coap_dtls_get_overhead(coap_session_t *session UNUSED) {
   return 0;
 }
 
+void *coap_tls_new_client_session(coap_session_t *session UNUSED) {
+  return NULL;
+}
+
+void *coap_tls_new_server_session(coap_session_t *session UNUSED) {
+  return NULL;
+}
+
 ssize_t coap_tls_write(coap_session_t *session UNUSED,
                        const uint8_t *data UNUSED,
                        size_t data_len UNUSED

src/coap_tinydtls.c

@@ -434,6 +434,40 @@ unsigned int coap_dtls_get_overhead(coap_session_t *session) {
   return 13 + 8 + 8;
 }
 
+#ifdef __GNUC__
+#define UNUSED __attribute__((unused))
+#else /* __GNUC__ */
+#define UNUSED
+#endif /* __GNUC__ */
+
+int coap_tls_is_supported(void) {
+  return 0;
+}
+
+void *coap_tls_new_client_session(coap_session_t *session UNUSED) {
+  return NULL;
+}
+
+void *coap_tls_new_server_session(coap_session_t *session UNUSED) {
+  return NULL;
+}
+
+ssize_t coap_tls_write(coap_session_t *session UNUSED,
+                       const uint8_t *data UNUSED,
+                       size_t data_len UNUSED
+) {
+  return -1;
+}
+
+ssize_t coap_tls_read(coap_session_t *session UNUSED,
+                      uint8_t *data UNUSED,
+                      size_t data_len UNUSED
+) {
+  return -1;
+}
+
+#undef UNUSED
+
 #else /* !HAVE_LIBTINYDTLS */
 
  /* make compilers happy that do not like empty modules */