genrandconfig: fix the case when outputdir is 'output'

When outputdir is 'output' (the default), genrandconfig didn't work
correctly because it expects the configfile in outputdir, while
Buildroot puts it in the buildroot directory.

Fix this by explicitly checking if outputdir == buildrootdir/output.
Because abspath is used for both paths, string comparison works
reliably.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>