buildroot

flex: only prevent the flex binary to be built for target, not for host

We do want to build the flex binary when building host-flex since it's
needed as a build dependency for other packages.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>