* live-build/ubuntu-cpc/functions: Add a function, teardown_mountpoint,
to reverse the work done in setup_mountpoint. Lack of this function
has forced users of setup_mountpoint to implement this separately
and the implementations have diverged. (LP: #1716992)
* live-build/ubuntu-cpc/functions: Remove umount_settle function.
The was only used where teardown_mountpoint was lacking.
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.
