URPC server wait defer instead of yield for lower CPU load

lib/aos/aos_urpc.c

@@ -17,6 +17,7 @@
 #include <aos/waitset_chan.h>
 #include <aos/aos_rpc.h>
 #include <spawn/rpc_server.h>
+#include <aos/deferred.h>
 
 extern coreid_t my_core_id;
 extern rpc_handler_t rpc_handlers[RPC_MTYPE_COUNT];
@@ -169,8 +170,8 @@ int urpc_server(void *arg) {
     while (true) {
         // wait until an rpc call arrives
         while(urpc->meta->call_in_progress == false) {
-            // yield the thread because there might be useful stuff to do...
-            thread_yield();
+            // sleep for a bit while there is nothing to do
+            barrelfish_usleep(100);
         }
 
         // memory barrier