qisys: add XDG base directory support
qisys: add XDG base directory support

This adds an optional dependency on pyxdg. Without pyxdg, qibuild will
use the same paths as before.

Change-Id: I264ddb5c4ebba92b98e62ad8fe4192dbeac8bfcd
Reviewed-on: http://gerrit.aldebaran.lan/54856
Reviewed-by: smartin <edaf33f3e53483e687d304aa81a03bb5a17b9ddf@aldebaran-robotics.com>
Reviewed-by: dmerejkowsky <1e4b13000fa8777df4cefd2fd8215187e3deb062@aldebaran-robotics.com>
Approved: nrubinstein <958004d5663b4fd0a0d26fef541880ee5b34c6ef@aldebaran-robotics.com>
Tested-by: nrubinstein <958004d5663b4fd0a0d26fef541880ee5b34c6ef@aldebaran-robotics.com>

aldebaran/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild