Bug#863815: When installed in USB stick, grub configuration use device name instead of uuid

[syl <syl@gresille.org>]

Package: debian-installer
Version:

Dear Maintainer,

I install Stretch from a USB stick to a USB stick (2 weeks ago).

After installation, I remove the installation USB stick and reboot. The
system don't start, I have an error after grub.
If I reboot and plug again the other USB stick, the system can start.

To fix it, I edit grub configuration:
I replace root=/dev/sdc1 by root=UUID=$uuid_of_the_root_partition