Thanks ade,
that was the missing link.

I already had the updated u-boot-update script, but was wondering about that "/var/tmp/bootmenu.cmd.default" error. (So used original version.)
But as I like to have the run attachboot as default, I just omit that /etc/default/bootmenu.item? Will the script run?
Or is there another/nicer way to have attached kernel as default with new u-boot? (Because I do not have an item like that. OK. could create one, but hen I would have it two times in list, right?)