Cosmetic: use symbolic constant STK_LEAVE_PROGMODE ('Q')

http://code.google.com/p/optiboot/issues/detail?id=55

optiboot/bootloaders/optiboot/optiboot.c

@@ -538,7 +538,7 @@ int main(void) {
       putch(SIGNATURE_1);
       putch(SIGNATURE_2);
     }
-    else if (ch == 'Q') {
+    else if (ch == STK_LEAVE_PROGMODE) { /* 'Q' */
       // Adaboot no-wait mod
       watchdogConfig(WATCHDOG_16MS);
       verifySpace();