Remove use of mapper call

Replace invocations of mapper call with a call to busctl and mapper
get-service.  This provides equivalent functionality but opens the
door for dropping a systemd patch we have carried for some time.

Change-Id: Iea0cce89341939b49b095d1f5010415578331d4d
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/common/recipes-phosphor/host/obmc-op-control-host/op-start-host@.service b/common/recipes-phosphor/host/obmc-op-control-host/op-start-host@.service
index 2617941..8703a43 100644
--- a/common/recipes-phosphor/host/obmc-op-control-host/op-start-host@.service
+++ b/common/recipes-phosphor/host/obmc-op-control-host/op-start-host@.service
@@ -14,8 +14,8 @@
 [Service]
 Type=oneshot
 RemainAfterExit=yes
-ExecStart=/usr/bin/env mapper call /org/openbmc/control/host%i org.openbmc.control.Host boot
-SyslogIdentifier=mapper
+ExecStart=/bin/sh -c "busctl call `mapper get-service /org/openbmc/control/host%i` /org/openbmc/control/host%i org.openbmc.control.Host boot"
+SyslogIdentifier=op-start-host
 
 [Install]
 WantedBy=obmc-host-start@%i.target