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.

drivers/net/Kconfig

@@ -40,7 +40,7 @@ config NET_RPMSG_PRIORITY
 
 config NET_RPMSG_STACKSIZE
 	int "net rpmsg stack size"
-	default 2048
+	default DEFAULT_TASK_STACKSIZE
 	---help---
 		The stack size allocated for the net rpmsg task.