make: force traversal into PKG's, same as normal modules
pkg: add USEPKG module "libfixmath"

This PR adds the USEPKG module "libfixmath".
It integrates https://code.google.com/p/libfixmath/ into RIOT, cmp. #1457.
Merge pull request #1460 from Kijewski/issue-1457

pkg: add USEPKG module "libfixmath"