core/legal-info: update the legal-info report header

In some cases, the toolchain sources are now recovered and available in
the legal-info output. So, adapt the header to use conditional instead
of an definitive negation.

Also update the part about saving the sources: it's not the license list
that defines whether sources are installed, but rather whether the
package is redistributable or not.

Update the header accordingly.

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