Fix issue where debconf was asking about the kernel config.

Fix issue where debconf was asking about the kernel config.

The chroot's apt.conf.d has a debconf file which tells apt
to run dpkg-preconfigure on all packages before installing.
We don't want to do this, especially with an automated build.
This CL changes the apt config used by install_packages.sh to
be self-contained and skip including the default configs by
using the APT_CONFIG evnironement variable.

The CL also cleans up some of the apt command lines as well
as getting rid of some ugly "$ROOT_FS_DIR/..".

[Bill Richardson, 2009-12-23 20:23:52]

LGTM