deconf: limit environment passed to deconf

this one is a bit more controversial.
in hwaf I do store a fair bit of non-os.environ stuff in the ctx.env (dicts of projects trees, stack of operations modules applied on the environment, etc...)

passing the full environment as in that callstack:

https://github.com/hwaf/worch/blob/master/orch/waffuncs.py#L57
https://github.com/hwaf/worch/blob/master/orch/pkgconf.py#L99

breaks hwaf.