Do not fail if ISO doesn't have grub.cfg inside efi.img

If an ISO doesn't support Secure Boot then the grub.cfg file doesn't
exist inside the efi.img, so we shouldn't continue with its execution.
Use search_file() instead which is unset if the file doesn't exist and
therefore execution gets skipped.

Follow up fix for commit 92ffc08

grml2usb

@@ -1760,7 +1760,7 @@ def handle_secure_boot(target, efi_img):
         logging.critical("Fatal: %s", error)
         sys.exit(1)
 
-    grub_cfg = efi_mountpoint + "/boot/grub/grub.cfg"
+    grub_cfg = search_file("grub.cfg", efi_mountpoint + "/boot/grub/")
     logging.debug("grub_cfg = %s" % grub_cfg)
     if not grub_cfg:
         logging.info(