systemd: add SMACK support option.

A new configuration option is available in systemd menu, to enable
support for SMACK.

For this feature to properly work, systemd requires attr (build
dependency, also used for other features) and smack (runtime dependency).

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>