boot: prompt retry to load polio.com

fat12boot.c

@@ -47,8 +47,12 @@ static void loadpolmon() {
         die("fi");
     }
 
-    if (fat12_readfile("POLMON  COM", kPolmonAddress)) {
+    while (fat12_readfile("POLIO   COM", kPolmonAddress)) {
-        die("pnf");
+        asm volatile (
+                "mov $00, %%ah      \n\t"
+                "int $0x16          \n\t"
+                ::: "ax"
+                );
     }
 
     jump(kPolmonAddress);