Running tinbuild2 with srcdir != builddir

If you run a *basic* (no gerrit stuff for instance) type of tinderbox
(or several) for the master branch using the old tinbuild2 script, and
want to use a separate srcdir (which now works fine for normal
builds), this patch might come in handy: http://pastebin.com/cKanrQiq
.

No, I am not going to push it, tinbuild2 is being obsoleted anyway.
This was just a stopgap measure until the shiny new tb script works
for my use case. And no, I don't "support" this. It just seems to work
for me. (I run tinbuild2 with options -p <profile> -m all
--srcdir=<srcdir> -w <sleeptime>)

--tml