Move cookbooks to subdirectory "chef"

Clarify the fact that we are running "chef" by
moving the "cookbooks" to subdirectory "chef".
In that way it's much easier to add different
provisoning later on.
Cleanup and moving udev Rule to cookbook "etc"

The customisation of the udev Rule needed for the ST-Link/V2 Programmer
is part of adapting "/etc". Therefore move it to the recipe "etc".
Further cleanup of cookbooks and '.gitignore'.
Adapt README.md regarding the Virtual Dev Machine.

Mention that the Virtual Development Machine is based on
Ubuntu 12.04. and clarify wording.