we don't have to leave empty space in our derivative images for packages
that have been downloaded/installed/removed. This normally isn't
relevant for the installed system, since the root filesystem will
auto-expand in place on the target disk, but lets us ship smaller
images.
* live-build/functions: also call 'apt-get update' after mounting the
blank /var/lib/apt.
lp:~rbalint/livecd-rootfs/livecd-rootfs-autopkgtest-fix to wrap kpartx
and trap spurious errors, since the problem is still being seen
intermittently on i386 despite us calling sync before kpartx.
