When update-grub is run, it will detect the disks in the build system.
Currently, we sed the grub configuration in places where this happens;
replace_grub_root_with_label moves that seddery in to a single callable
location to avoid ending up with multiple versions of it.
(It's worth noting that we also do this in 999-cpc-fixes.chroot, but as our
functions file isn't available within the chroot we don't use it there.)
from a launchpad-buildd-like setup, which passes ARCH and SUBARCH in the
environment, because auto/config and auto/build both rely on this. So
don't scatter dpkg --print-architecture calls throughout, especially
when many of these are not cross-build-aware.
