tools: move the builddir lookup function out to a separate file

We want to use this from the tests as well soon, so let's move it to a more
generic location. This also changes the API to be slightly more sensible, a
free() is the same cost (and safer) than passing a static buffer in and hoping
we didn't get the size wrong.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>