Bug#850763: Option for build directory is needed

[Alf Gaida]

> Can you try setting those environment variables and let me know if it 
works?
> If it does then this is just a docfix.

% sudo TMP=. lwr
DEBUG environment: TMP=.
DEBUG vmdebootstrap command: ... --squash=./tmp5fr6Ev/live ...
looks good so far

Cheers Alf