Use local apt cache instead of local_repo when mastering an image.

Use local apt cache instead of local_repo when mastering an image.

This change switches to mastering an image without using debootstrap.
We turn on the previously experimental bits that install a small
set of packages manually before handing things over to apt. In both
cases we use apt to download the packages so that it can populate
it's local package cache.

With this change we will no longer depend on the local_repo when
mastering an image. Developers will not have to rebuild their
local repo when repo_list_image.txt changes. Instead we will use
and lazy-fill the local apt-cache. The first time you build_image.sh
it will be slow since it needs to download the packages. Subsequent
runs should be as before since it will use the local cache. If
packages are added they should be lazily fetched in the next image
build.

Until we have a fully populated external mirror, developers will
still have to add packages to repo_list_image.txt. Also, until
make_chroot is switched over to use the external repo then
developers will have to redo their local repo when the
repo_list_dev.txt changes.

[Bill Richardson, 2010-01-08 16:05:09]

LGTM