[init] Remove concept of "shutdown exit flags"

Remove the concept of shutdown exit flags, and replace it with a
counter used to keep track of exposed interfaces that require devices
to remain active.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/image/efi_image.c

@@ -39,7 +39,7 @@ static EFI_EVENT efi_shutdown_event;
  */
 static EFIAPI void efi_shutdown_hook ( EFI_EVENT event __unused,
 				       void *context __unused ) {
-	shutdown ( SHUTDOWN_BOOT );
+	shutdown_boot();
 }
 
 /**