systemd: Add host identifier to service description

On a multi-host system, there is no identifier for the post code manager
service, so you end up with console messages such as:

```
         Starting Post code manager...
         Starting Post code manager...
         Starting Post code manager...
         Starting Post code manager...
         Starting Post code manager...
         Starting Post code manager...
         Starting Post code manager...
         Starting Post code manager...
```

Add an identifier to the templated service file so that these instances
are differentiated.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I63c0a5d80f4870314805b8f9ff07782be9fe3faf
diff --git a/service_files/xyz.openbmc_project.State.Boot.PostCode@.service b/service_files/xyz.openbmc_project.State.Boot.PostCode@.service
index 1118434..db21c21 100644
--- a/service_files/xyz.openbmc_project.State.Boot.PostCode@.service
+++ b/service_files/xyz.openbmc_project.State.Boot.PostCode@.service
@@ -1,5 +1,5 @@
 [Unit]
-Description=Post code manager
+Description=Post code manager (host %i)
 
 [Service]
 ExecStart=/usr/bin/env post-code-manager --host %i