buildroot

core/legal-info: also save extra downloads

Some packages, like perl, download extra files that end up as part of
the source that Buildroot builds. Up until now, those files were not
saved in the legal-info output.

Add those files to the legal-info output.

The unfortunate side-effect is that we will also save the secondary
archive for the external blackfin toolchains; however, we already do
save the binary release of some external toolchains when they do not
provide actual source archives.

This is inherently bad, as those are not source archives, but solving
this is a bigger concern, for another series...

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