Add current time to posix RNG seed. (#730)

examples/platforms/posix/random.c

@@ -34,6 +34,7 @@
  *   This implementation is not a true random number generator and does @em satisfy the Thread requirements.
  */
 
+#include <time.h>
 #include <openthread-types.h>
 
 #include <common/code_utils.hpp>
@@ -44,7 +45,8 @@ static uint32_t s_state = 1;
 
 void posixRandomInit(void)
 {
-    s_state = NODE_ID;
+    // Multiplying NODE_ID assures that no two nodes gets the same seed within an hour.
+    s_state = (uint32_t)time(NULL) + (3600 * NODE_ID);
 }
 
 uint32_t otPlatRandomGet(void)