legal-info: save license files even for local or overridden packages

Even if we do not save the sources for local or overridden packages because
it is too complex, we can still quite easily save the license files.

Also, having the license files is a very important part of complying with
the licenses.

Move the copy of license files out of the non-local, non-overridden package
case, but still in the case where packages have a _SOURCE defined, to
avoid catching packages bundled in Buildroot (eg. mkpasswd et al.)

Reported-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Fabio Porcedda <fabio.porcedda@gmail.com>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>