Allow loader.efi to identify non-standard boot setup

PATH_BOOTABLE_TOKEN can be set to a non-standard
path that identifies a device as bootable.

Reviewed by: kevans, bcran
Differential Revision:  https://reviews.freebsd.org/D22062

stand/efi/loader/main.c

@@ -239,6 +239,9 @@ sanity_check_currdev(void)
 	struct stat st;
 
 	return (stat(PATH_DEFAULTS_LOADER_CONF, &st) == 0 ||
+#ifdef PATH_BOOTABLE_TOKEN
+	    stat(PATH_BOOTABLE_TOKEN, &st) == 0 || /* non-standard layout */
+#endif
 	    stat(PATH_KERNEL, &st) == 0);
 }