test-pkg: 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.

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