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/users/files/obmc-phosphor-user.service b/common/recipes-phosphor/users/files/obmc-phosphor-user.service
index 7c1ffce..e11a0a9 100644
--- a/common/recipes-phosphor/users/files/obmc-phosphor-user.service
+++ b/common/recipes-phosphor/users/files/obmc-phosphor-user.service
@@ -2,9 +2,9 @@
 Description=Phosphor OpenBMC user management daemon
 
 [Service]
-ExecStart=/usr/sbin/obmc-phosphor-userd
+ExecStart={sbindir}/obmc-phosphor-userd
 Environment="PYTHONUNBUFFERED=1"
 Restart=always
 
 [Install]
-WantedBy=multi-user.target
+WantedBy={SYSTEMD_DEFAULT_TARGET}