Don't add startx notice to motd

Our image won't have X, so this is unnecessary
Reduce stage3 package list.

Removing packages that aren't needed for a minimal install.
Remove NOOBS stuff.

We're not going to be using NOOBS
Remove tweaks to default .bashrc

Prefer to stick with the stock debian one.  Tweaks can be done later by
a config management tool, and shouldn't be part of the base image
Fix network subnet in qemu image.

It was defaulting to a class A subnet, which let to conflicts with other
10.* addresses in the host network.
Update debian and raspbian mirrors.

Use the mirrordirectors for both to always get a nearby mirror
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.