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>

Config.in.legacy

@@ -147,6 +147,7 @@ comment "Legacy options removed in 2016.08"
 
 config BR2_PACKAGE_SYSTEMD_COMPAT
 	bool "systemd compatibility libraries have been removed"
+	select BR2_LEGACY
 	help
 	  The systemd option to enable the compatibility libraries has
 	  been removed. Theses libraries have been useless since a few