grub2: If using --sysroot, run in chroot

In Anaconda, we're using "ostree admin --sysroot=/mnt/sysimage
instutil set-kargs", and it was working before, but newer versions of
lorax strip out /etc/system-release which grub2 wants.

That was wrong anyways as we want the /etc/system-release from the
target root.

(Man, grub2 sucks...give me a declarative config file format I can just
 write)

https://bugzilla.gnome.org/show_bug.cgi?id=740697