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.
Use DEBIAN_FRONTEND=noninteractive

Prevents errors about loading up other interactive frontends
Install stage3 packages in one batch.

Much quicker this way as it only has to resolve dependencies once.
Allow hostname to be set from dhcp.

Requires workaround for dhcp client scripts. See
http://blog.schlomo.schapiro.org/2013/11/setting-hostname-from-dhcp-in-debian.html
for more details.
Remove stage4 scripts.

We're only producing a minimal image, so these are unnecessary.
Tweak network conf for static wireless config

We're aiming these images are a more server-style deployment where the
wireless network will be part of the system config, not dynamic.
wheezy-stage2: remove kgbdoc from cmdline.txt

We have no need for the kernel debugger.
Explicitly unmount partitions after copying /boot

When this was left to universal_cleanup there appeared to be a race
condition where boot would still be busy when attempting to unmount it.
The unmount would therefore fail.  This led to the nbd being
disconnected without the writes to the boot partition being committed,
resulting in an empty /boot in the resulting image.