I have been experimenting with different versions of 40_custom with the Crux information either before or after the "exec tail -n +3 $0" statement (as the included comment does not make it clear which way it should go). Both caused update-grub to report syntax errors (different ones!). But I have found a temporary work-around.
What is going wrong is that the "echo", "cat <<EOF" and "EOF" lines in 40_custom are being put into grub.cfg instead of being treated as working instructions. When I edited them out of grub.cfg.new by hand and renamed the resultant file as grub.cfg, it gave me a good boot.
