As noted inline, the switch to "boot loader spec" grub entries breaks
our setting of the root device. This happened some time ago, and it's
not 100% clear to me why our existing Fedora builds haven't broken on
this. However, the new containerfile based builds do seem to be
hitting this.
Disable it for now.
Change-Id: Ia3472947799bb35ffccfa92937cdd0d68b12a25c