genrandconfig: get configs from in-tree toolchain-configs.csv

Now we have the toolchain config fragments in the buildroot directory
itself, it is no longer necessary to fetch it from the toolchain URL.

The --toolchains-url option is renamed to --toolchains-csv.

The paths in the toolchains_csv file should be either absolute, or
relative to buildrootdir.

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-csv", kwargs['toolchains_csv']],
                    stdout=devnull, stderr=log)

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