repo: Default bootloader to zipl on s390x

This will allow us to drop code like
https://github.com/coreos/coreos-assembler/blob/87fc693c115eae1e7ff0e1621ce26c9167af6e84/src/create_disk.sh#L503
which is really just unnecessary since there aren't any other
bootloaders we care about on this architecture.

src/libostree/ostree-repo.c

@@ -3298,9 +3298,14 @@ static gboolean
 reload_sysroot_config (OstreeRepo *self, GCancellable *cancellable, GError **error)
 {
   g_autofree char *bootloader = NULL;
+#if !defined(__s390x__)
+  const char *default_bootloader = "auto";
+#else
+  const char *default_bootloader = "zipl";
+#endif
 
   if (!ot_keyfile_get_value_with_default_group_optional (self->config, "sysroot", "bootloader",
-                                                         "auto", &bootloader, error))
+                                                         default_bootloader, &bootloader, error))
     return FALSE;
 
   /* TODO: possibly later add support for specifying a generic bootloader