legal-info: fix detection of "override" packages

legal-info: fix detection of "override" packages

The ifneq ($$($(2)_SITE_METHOD),override) test will always match
because SITE_METHOD is never set to "override". The result is that
some packages are downloaded and extracted unnecessarily, although
they are (correctly) not saved in output/legal-info/.

Fix by simply testing $(2)_OVERRIDE_SRCDIR instead: when empty, it
indicates that the packages is neither a "local" site-method package
nor a package that has its source directory overridden.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>