qibuild: move build folder name generation into build_config

Allows the function to be reused in more place... like qipkg

Change-Id: Ibde35ca7e03f2da3dbe2172f34d18342cf3cb316
Reviewed-on: http://gerrit.aldebaran.lan/31938
Tested-by: gerrit
Reviewed-by: vbarbaresi <vbarbaresi@aldebaran-robotics.com>