bare-metal-gpio-monitor: don't start on non-bm mode

This adds a condition check for the bm flag. So that the service will
not take effect on non-bm mode.

Change-Id: I0d15247a6e05aa5edf93468793cd74bd7d9ea158
Signed-off-by: Yuxiao Zhang <yuxiaozhang@google.com>
diff --git a/subprojects/bare-metal-host-monitor/host-gpio-monitor.service.in b/subprojects/bare-metal-host-monitor/host-gpio-monitor.service.in
index 3da5f1d..a0bee9e 100644
--- a/subprojects/bare-metal-host-monitor/host-gpio-monitor.service.in
+++ b/subprojects/bare-metal-host-monitor/host-gpio-monitor.service.in
@@ -1,5 +1,6 @@
 [Unit]
 Description=gBMC bare-metal GPIO  monitoring
+ConditionPathExists=/var/google/config-package/enable-bm.flag
 After=phosphor-ipmi-host.service
 
 [Service]