universe: allow moving build logs to TINDERBOX_LOGDIR

[siva]

Summary

This allows make universe or make tinderbox
to build from a read-only src tree.

[jhb]

I actually find it really convenient that the log files are in .CURDIR.

[siva]

I think the convenience is nice for sure, but the flexibility of choice is reduced. If anything, maybe I can add a variable similar to FAILFILE that is configurable so that users can choose where it goes?

I do like the idea of being able to build from a read-only src tree.

[siva]

How about this instead? This keeps the old behaviour by default
for backwards compatibility.

[delphij]

Looks fine to me.