core/legal-info: install source archives in their own sub-dir

core/legal-info: install source archives in their own sub-dir

Currently, we put all source archives side-by-side in the same
directory.

Since we're about to also save individual patches that were applied
on those sources, we don't want to make that directory a complete
mess of unassorted files.

So, we install each source archive in its own sub-directory, where
we'll later store the patches too. Store that location in a variable,
so it can be re-used later on (to install patches in a future commit).

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