postfix: start the postfix daemon by default when init is systemd

postfix should be started by default, so that the root user can
receive mail, and if init is not systemd, it is started by default

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>