Add config option to set boot menu delay time.

Add CONFIG_BOOTMENU_WAIT option with delay time (in milliseconds).
diff --git a/src/boot.c b/src/boot.c
index 45d808f..a7795a1 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -205,7 +205,7 @@
 
     printf("Press F12 for boot menu.\n\n");
 
-    int scan_code = get_keystroke(2500);
+    int scan_code = get_keystroke(CONFIG_BOOTMENU_WAIT);
     if (scan_code != 0x86)
         /* not F12 */
         return;