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/rest-dbus/files/rest-dbus.service b/common/recipes-phosphor/rest-dbus/files/rest-dbus.service
index d3bddab..d22d2a2 100644
--- a/common/recipes-phosphor/rest-dbus/files/rest-dbus.service
+++ b/common/recipes-phosphor/rest-dbus/files/rest-dbus.service
@@ -3,5 +3,5 @@
 
 [Service]
 Restart=always
-ExecStart=/usr/sbin/rest-dbus
+ExecStart={sbindir}/rest-dbus
 Environment="PYTHONUNBUFFERED=1"