Fixed hello sleep

usr/hello/hello.c

@@ -15,7 +15,6 @@
 
 
 #include <stdio.h>
-#include <unistd.h>
 #include <string.h>
 
 #include <aos/aos.h>
@@ -35,8 +34,8 @@ int main(int argc, char *argv[])
     if (argc > 1 && !strncmp(argv[1], HELLO_CATCH_COMMAND, sizeof(HELLO_CATCH_COMMAND))) {
         while(1) {
             printf("Catch me if you can!\n");
-            // wait for 1 second
-            sleep(1);
+            // wait a bit
+            for(volatile int j = 0; j < 500000000; ++j);
         }
     }