Comment 2 for bug 1688424

Studied /etc/grub.d/10_linux a little bit deeper and found out, that the 'grub-mkrelpath /' is probably expected, but the way, how $rpool gets determined is buggy (and in this case it results into an empty string). Attaching a patch, which resolves the problem for me (just deduces it from whatever is mounted on / right now).