native: optionally exit() after last thread has ended

Used when compiled with NATIVE_AUTO_EXIT defined.

cpu/native/README

@@ -128,3 +128,11 @@ is established.
 
 Socket redirection is only available when the uart module has been
 compiled in.
+
+COMPILE TIME OPTIONS
+====================
+
+Compile with
+    CFLAGS=-DNATIVE_AUTO_EXIT make
+
+to exit the riot core after the last thread has exited.

cpu/native/native_cpu.c

@@ -151,6 +151,13 @@ void isr_cpu_switch_context_exit(void)
 
 void cpu_switch_context_exit()
 {
+#ifdef NATIVE_AUTO_EXIT
+    if (num_tasks <= 1) {
+        DEBUG("cpu_switch_context_exit(): last task has ended. exiting.\n");
+        exit(EXIT_SUCCESS);
+    }
+#endif
+
     if (_native_in_isr == 0) {
         dINT();
         _native_in_isr = 1;