enable local debian repositories for use with elbe

if you want to use a debian repository on your local machine (the box you
are typing the elbe commands). You can now use sth. like
deb http://LOCALMACHINE/my-repo wheezy main
in the url-list.

LOCALMACHINE will be replaced by localhost for elbe create and
buildchroot.py. For the debian-installer and inside the buildenv 10.0.2.2
will be used. This is the IP of your local machine can be reached inside
qemu.

Signed-off-by: Manuel Traut <manut@linutronix.de>
if a proxy is configured; 10.0.2.2 should be ignored

Typically the LOCALMACHINE / 10.0.2.2 will not be configured on any proxy.
This patch adds an apt.conf template that will be used inside the buildenv
and the debian-installer to skip the use of a proxy for this IP.

Signed-off-by: Manuel Traut <manut@linutronix.de>
add source repositories the virtapt cache

Signed-off-by: Manuel Traut <manut@linutronix.de>
disallow the use of localhost inside the url-list

if anyone wants to use a debian repository on the local machine,
LOCALMACHINE should be used instead of localhost. It will be replaced
by elbe - either by localhost for the code running on the local machine
or by 10.0.2.2 for the code running inside qemu.