Improve BUILDDIR handling with symlinks

If BUILDDIR is a local symlink to another directory in the same parent
directory (out -> out.angler), then using out and .. as relative paths
to get back and forth work.

But if BUILDDIR is a symlink to another directory altogether (out ->
/mnt/sdd/out.master), then we shouldn't be relying on relative paths (so
that the source directory can still be moved).

Change-Id: I946c8116090410ab2b935eafba9b6e96f5f2f1dd
3 files changed