Add support for an apt proxy.

If set, this gets used for bootstrapping the image, and also configured
inside the image for future package installs.
Don't install raspi-config

Unneeded - we can do all this config without it.
Remove unused/redundant script sections
Reduce image size

We're only creating a minimal install, so it doean't need to be as big
as for a full image.
Configure timezone in stage2

Setting timezone values in debconf in stage3 is no good because tzdata
is already installed by then, therefore the changes don't take effect.
Fix sudo in image export

Because attach_image_to_nbd calls sudo in the background, this can lead
to mixed up output when prompting for passwords.  Calling sudo -v
beforehand prevents this.
Update rpi-issue to reference forked repo.