os-builder: define a variable for the config dir?

[Daniel Drake]

On Thu, 2011-01-06 at 11:47 -0500, Martin Langhoff wrote:
> Turns out ConfigParser handles this just fine -- and it was already
> trying to interpolate anything with a % in there. We can just drive it
> -- sample patch attached.
> 
> If we don't want any interpolation (it was a surprise to me that
> ConfigParser interpolates by default) -- we may need to get() the raw
> values instead.

It was a semi-conscious decision to have interpolation available, and
its something I've used on a couple of deployments. You also found a use
for it, so I think it should stay. Pushed your patch with some
documentation added, thanks.

Daniel