sysvinit: depend on SysV selected as init system.

In order to prevent sysvinit and systemd to be selected at the same
time, sysvinit can now only be selected if SysV is chosen as init
system.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>