add user threading macro definition

wolfssl/wolfcrypt/types.h

@@ -1400,6 +1400,20 @@ typedef struct w64wrapper {
  #endif
  typedef void* THREAD_TYPE;
  #define WOLFSSL_THREAD
+ #elif defined(WOLFSSL_USER_THREADING)
+ /* User can define user specific threading types
+ * THREAD_RETURN
+ * TREAD_TYPE
+ * WOLFSSL_THREAD
+ * e.g.
+ * typedef unsigned int THREAD_RETURN;
+ * typedef size_t THREAD_TYPE;
+ * #define WOLFSSL_THREAD void
+ *
+ * User can aslo implement user own wolfSSL_NewThread(),
+ * wolfSSL_JoinThread() and CND(s) if want.
+ * Otherwise, those functions are omitted.
+ */
  #elif defined(WOLFSSL_MDK_ARM) || defined(WOLFSSL_KEIL_TCP_NET) || \
  defined(FREESCALE_MQX)
  typedef unsigned int THREAD_RETURN;