buildroot

legacy: properly handle systemd compatibility libs

The legacy symbol for systemd compatibility libraries was not selecting
the BR2_LEGACY symbol, so the buld-time check would not kick in.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>