Add -u to usage message.
[pintos-anon] / src /
drwxr-xr-x   ..
-rw-r--r-- 364 Makefile
-rw-r--r-- 3856 Makefile.build
-rw-r--r-- 275 Makefile.kernel
-rw-r--r-- 1231 TODO
drwxr-xr-x - devices
drwxr-xr-x - filesys
drwxr-xr-x - lib
drwxr-xr-x - misc
drwxr-xr-x - threads
drwxr-xr-x - userprog
drwxr-xr-x - utils