button-handler:  Add button-handler service file

This service will start the button-handler app on the way to
standby after the mapper and the buttons app.

Tested:  Built an image and saw it start.

Change-Id: I4d08fa64f2a15486a4ccd33eaf7c6a89a281594e
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/service_files/phosphor-button-handler.service b/service_files/phosphor-button-handler.service
new file mode 100644
index 0000000..3771f62
--- /dev/null
+++ b/service_files/phosphor-button-handler.service
@@ -0,0 +1,15 @@
+[Unit]
+Description=Phosphor Button Handler
+Wants=xyz.openbmc_project.Chassis.Buttons.service
+After=xyz.openbmc_project.Chassis.Buttons.service
+Wants=obmc-mapper.target
+After=obmc-mapper.target
+
+[Service]
+Restart=always
+ExecStart=/usr/bin/env button-handler
+SyslogIdentifier=button-handler
+
+[Install]
+WantedBy=obmc-standby.target
+