Comment # 2 on bug 1159887 from Michael Chang

(In reply to Josef Reidinger from comment #1)
> that make sense for me.
> 
> Michael - just quick question, how to reset already set value that we set by
> "grub-set-default" to use really the last one?

I believed the simple configuration [1] GRUB_SAVEDEFAULT is what you are
looking for. And if you read the manual thoroughly, you'll see the caveat in
the end. 

"This option relies on the environment block, which may not be available in all
situations"

That is also saying we may have to detect either of the zfs, lvm and mdadm is
in use by /boot and pop up warning whenever writable environment block not
available.

[1]
https://www.gnu.org/software/grub/manual/grub/grub.html#Simple-configuration

Thanks.