Compatibility fix for Design Assets Management

design.sh uses `readlink -m` to determine canonical path of git dir,
which is not available on all *nix systems (including Darwin).

Instead, use `pwd -P` to get canonical directory path in a more
cross-platform compatible way.

This is a change to an actual script rather than just a test, and it
should make the Design Assets Management functionality of scm_breeze
now function properly on MacOSX.