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/meta-phosphor/common/recipes-phosphor/dbus/obmc-mapper/org.openbmc.ObjectMapper.service b/meta-phosphor/common/recipes-phosphor/dbus/obmc-mapper/org.openbmc.ObjectMapper.service
index b397e2f..1f1e832 100644
--- a/meta-phosphor/common/recipes-phosphor/dbus/obmc-mapper/org.openbmc.ObjectMapper.service
+++ b/meta-phosphor/common/recipes-phosphor/dbus/obmc-mapper/org.openbmc.ObjectMapper.service
@@ -5,10 +5,10 @@
 [Service]
 Restart=always
 Type=dbus
-ExecStart=/usr/sbin/phosphor-mapper
-BusName=org.openbmc.ObjectMapper
+ExecStart={sbindir}/phosphor-mapper
+BusName={BUSNAME}
 TimeoutStartSec=300
 Environment="PYTHONUNBUFFERED=1"
 
 [Install]
-WantedBy=multi-user.target
+WantedBy={SYSTEMD_DEFAULT_TARGET}