make legal-info: fails with OVERRIDE_SRCDIR

There is a check for OVERRIDE_SRCDIR in pkg-generic.mk that is
supposed to produce a warning when OVERRIDE_SRCDIR is active.
This does not work and instead the whole make terminates with
an error message.

This patch changes the check for active OVERRIDE_SRCDIR so that
it works as expected.

Signed-off-by: Stephan Hoffmann <sho@relinux.de>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>