fsl-image-machine-test: Add GPU external tools

The packagegroup-fsl-tools-gpu-external brings glmark2 and mesa-demos,
when possible, and those are specially important for GPU test so being
important to be included in the machine-test image too.

Change-Id: Ifdf59d6a5b08540a34b8e38cfc3e1582e67b3c09
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>