Introduce CONFIG_DEFAULT_TASK_STACKSIZE

* The appropriate size of stack varies among archs.
  E.g. for 64-bit sim, 2048 is way too small, especially when the task
  happens to use host OS functionalities.
  I plan to allow an arch provide its own default.

* I plan to use this to replace hardcoded "STACKSIZE = 2048" in APPDIR.

boards/sim/sim/sim/Kconfig

@@ -16,7 +16,7 @@ comment "NX Server Options"
 
 config SIM_LISTENER_STACKSIZE
 	int "NX Server/Listener Stack Size"
-	default 2048
+	default DEFAULT_TASK_STACKSIZE
 	---help---
 		The stacksize to use when creating the NX server.  Default 2048