Fix GrubSetup.sh To Better Account For Non-LVM root=

[ferricoxide]

Problem Description:

When generating new "recovery" AMIs, the GrubSetup.sh currently sets the root= token to the chroot-built device's real devnode rather than properly referencing its LABEL value

Expected Behavior:

The root= tokens in the chroot()ed /boot/grub2/grub.cfg should be set to root=LABEL=<LABEL>

Actual Behavior:

The root= tokens in the chroot()ed /boot/grub2/grub.cfg are set to root=/chroot/devnode

(Detailed) Steps to reproduce:

Run GrubSetup.sh to create non-LVM'ed AMI