utils/test-pkg: add option to use an alternate toolchains CSV file

utils/test-pkg: add option to use an alternate toolchains CSV file

When testing a package with test-pkg, it may be useful to override the
set of toolchains used. For example:
- to test with toolchains used in your company;
- to test against a subset that is known to be problematic;
- to use only toolchains you already have available locally when you
  have no network access.

Add an option to use an alternate CSV file containing the config
fragments of toolchains to try.

Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>