Use systemd substitutions

Make use of the new substitution feature in the Phosphor
systemd bitbake class.

Change-Id: I2db85347809ef557a53e92af3ffcf17c8003b3ce
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/common/recipes-phosphor/events/files/obmc-phosphor-event.service b/common/recipes-phosphor/events/files/obmc-phosphor-event.service
index a93fce0..5aadcc0 100644
--- a/common/recipes-phosphor/events/files/obmc-phosphor-event.service
+++ b/common/recipes-phosphor/events/files/obmc-phosphor-event.service
@@ -4,7 +4,7 @@
 [Service]
 EnvironmentFile=-/etc/default/eventd/eventd.conf
 Restart=always
-ExecStart=/usr/sbin/obmc-phosphor-eventd $OPTIONS
+ExecStart={sbindir}/obmc-phosphor-eventd $OPTIONS
 
 [Install]
-WantedBy=multi-user.target
+WantedBy={SYSTEMD_DEFAULT_TARGET}