Let's do the same as memtest86+-4.20 does: test for presence of
/boot/grub/grub.cfg, if the file exists (and running some other
checks) we can execute update-grub, otherwise don't care....
This is supposed to address the annoying:
| /usr/sbin/grub-probe: error: cannot find a device for / (is /dev mounted?).
issue which is showing up in minimal chroots on package removal.
Closes: #660588