utils/genrandconfig: add aufs-util handling

Add a custom case to make sure that a random configuration with an empty
version for aufs-util doesn't fail.

Fixes:
 - http://autobuild.buildroot.org/results/e242cf66a02983bcf6e95b37f8e458bd18aee683



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>