Maintainer scripts: test for presence of grub.cfg, don't execute update-grub otherwise

Maintainer scripts: test for presence of grub.cfg, don't execute update-grub otherwise

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