utils/genrandconfig: only do reproducible builds with diffoscope

utils/genrandconfig: only do reproducible builds with diffoscope

After a few weeks of running reproducible builds in the autobuilders,
we found out that such builds without a diffoscope analysis are pretty
useless: the cmp on the tarballs doesn't help us fix the
reproducibility issue.

So, let's only do reproducible builds when diffoscope is available.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Atharva Lele <itsatharva@gmail.com>
Acked-by: Atharva Lele <itsatharva@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>