Add possibility to build image without web-interface

[jbaiter]

To quote @matti-kariluoma:

A raspberry pi stack that is low-tech (no external laptop, router, etc.) and dumps images directly to an attached usb stick.

@markvdb:

I had a prototype running in the past (Debian based). Gunirus has the board and the root file system that was on it. He will upload some of the relevant files from my old experiment.

• Automounting USB mass storage devices itself is not that difficult. See https://wiki.debian.org/AutoFs‎ .
• Trickier thing: FAT32 usb mass storage devices are easily corrupted if not unmounted properly. iirc, I used a beeping sound on the cameras to tell when it was safe to remove the stick.

This issue was copied from an earler "raspberry-pi-pauper-build" repository, which is now removed, to avoid confusion and code duplication.

[markvdb]

Is this one still valid? Do we still want this?

[markvdb]

The save to usb part is already implemented in a superior way by jbaiter using dbus python bindings and udisks. Not closing yet in case someone wants to work on this in the (far?) future. If so, this would probably require more feedback to the camera screens, which would mean dependency on CHDK. Messy...