Switch ok with debug mode apps, implement sys_exit correctly later.

os/src/syscall/process.rs

@@ -1,6 +1,12 @@
-use crate::batch::run_next_app;
+use crate::task::switch_to_next_task;
 
 pub fn sys_exit(xstate: i32) -> ! {
     println!("[kernel] Application exited with code {}", xstate);
-    run_next_app()
+    //run_next_app()
+    panic!("[kernel] first exit!");
+}
+
+pub fn sys_yield() -> isize {
+    switch_to_next_task();
+    0
 }