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/fans/files/obmc-phosphor-fand.service b/common/recipes-phosphor/fans/files/obmc-phosphor-fand.service
index 6698e12..9008954 100644
--- a/common/recipes-phosphor/fans/files/obmc-phosphor-fand.service
+++ b/common/recipes-phosphor/fans/files/obmc-phosphor-fand.service
@@ -3,7 +3,7 @@
 
 [Service]
 Restart=always
-ExecStart=/usr/sbin/obmc-phosphor-fand
+ExecStart={sbindir}/obmc-phosphor-fand
 
 [Install]
-WantedBy=multi-user.target
+WantedBy={SYSTEMD_DEFAULT_TARGET}