Revert "build: change waf URL to ftp://"
Revert "build: change waf URL to ftp://"

This reverts commit 30ed8d1c7942b6fd6c6a86451457513ea995e848.

Looks like the http:// URL works again (and ftp:// doesn't).

jwessel/pflask,jwessel/pflask,gdm85/pflask,gdm85/pflask,ghedo/pflask,ghedo/pflask