genrandconfig: pass outputdir and buildrootdir as arguments

The --instance argument is just an artifact of genrandconfig's
history as part of autobuild-run. It is much more logical to pass
the output directory and the buildroot directory as arguments, with
sane defaults.

This also allows us to remove the hack of creating a symlink in the
instance directory if it doesn't exist yet.

Note that the default outputdir 'output' doesn't work yet, because in
that case Buildroot will put the config file in the buildroot directory
instead of the output directory. This will be fixed in a follow-up
patch.

After this change, the script should be called from autobuild-run as:

    subprocess.call([os.path.join(srcdir, "utils/genrandconfig"),
                     "-o", outputdir, "-b", srcdir,
                     "--toolchains-url", kwargs['toolchains_url']],
                    stdout=devnull, stderr=log)

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