utils/genrandconfig: new script

This script will be used by the autobuild-run script to generate the
configuration to test. It is put in the utils directory because it can
also be called directly to allow users to test things.

For now, it is a direct copy of the relevant functions from the
autobuild-run script. The only changes are:
- unneeded import statements are removed;
- code/decode wrappers are limited to decode_byte_list;
- __main__ handling is added.

For now, the only supported arguments are the ones needed for
autobuild-run.  Follow-up patches will refactor things and also change
the way the script is called. In this version, it can be called from the
autobuild-run script as:

  subprocess.call([os.path.join(srcdir, "utils/genrandconfig"),
                   "-i", str(kwargs['instance']),
                   "--toolchains-url", kwargs['toolchains_url']],
                  stdout=log, stderr=log)

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