legal-info: extract even no-redistribute packages

Currently, if a package is marked _REDISTRIBUTE = NO, then legal-info
will not try to extract it first.

If that package also declares some _LICENSE_FILES, legal-info fails
if it is the only action we're trying to run:

    $ cat defconfig
    BR2_arm=y
    BR2_TOOLCHAIN_BUILDROOT_EGLIBC=y
    BR2_PACKAGE_LIBFSLCODEC=y
    $ make BR2_DEFCONFIG=$(pwd)/defconfig defconfig
    $ make legal-info
    [--SNIP--]
    cat: /home/ymorin/dev/buildroot/O/build/libfslcodec-3.5.7-1.0.0/EULA: No such file or directory

Fix this by always having legal-info extract the archives if one or
more _LICENSE_FILES are specified.

We do this for all types of packages: overridden, local or 'normal'
remote packages. Even though we do not save the sources for the
overridden or local packages, we need to save their licensing info,
so we need to extract them.

This implies that we now need only PKG-source, not PKG-extract anymore,
as a dependency of legal-info for packages we want to save (ie.
redistributable, non-local and non-overriden packages.)

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>