Load app from sdcard on K210, but panicked on qemu.

os/src/fs/stdio.rs

@@ -9,6 +9,8 @@ pub struct Stdin;
 pub struct Stdout;
 
 impl File for Stdin {
+    fn readable(&self) -> bool { true }
+    fn writable(&self) -> bool { false }
     fn read(&self, mut user_buf: UserBuffer) -> usize {
         assert_eq!(user_buf.len(), 1);
         // busy loop
@@ -33,6 +35,8 @@ impl File for Stdin {
 }
 
 impl File for Stdout {
+    fn readable(&self) -> bool { false }
+    fn writable(&self) -> bool { true }
     fn read(&self, _user_buf: UserBuffer) -> usize{
         panic!("Cannot read from stdout!");
     }