meta-phosphor: make discovery bbclass use override syntax

In the spirit of consistency with the new upstream override
syntax, change phosphor-discovery-service.bbclass to use it
and make tree-wide changes as appropriate.

After the change checked on Witherspoon on QEMU:
```
root@witherspoon:/etc/avahi/services# ls
obmc_console.service  obmc_redfish.service  obmc_rest.service
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ib1fe3cce57a0130378af789abd83b457e0c3a318
diff --git a/meta-phosphor/classes/obmc-phosphor-discovery-service.bbclass b/meta-phosphor/classes/obmc-phosphor-discovery-service.bbclass
index 4afd9e0..5b12faa 100644
--- a/meta-phosphor/classes/obmc-phosphor-discovery-service.bbclass
+++ b/meta-phosphor/classes/obmc-phosphor-discovery-service.bbclass
@@ -24,7 +24,7 @@
 
 
     for pkg in listvar_to_list(d, 'DISCOVERY_SVC_PACKAGES'):
-        for service in listvar_to_list(d, 'REGISTERED_SERVICES_%s' % pkg):
+        for service in listvar_to_list(d, 'REGISTERED_SERVICES:%s' % pkg):
             if avahi_enabled:
                 set_doappend(d, 'RRECOMMENDS:%s' % pkg, 'avahi-daemon')
                 svc_name, svc_type, svc_port, svc_txt_data = service.split(':')
@@ -95,7 +95,7 @@
             fd.write('%s %s %s' % (service_name, service_type, service_port))
 
     def register_services(d,pkg):
-        for service in listvar_to_list(d, 'REGISTERED_SERVICES_%s' % pkg):
+        for service in listvar_to_list(d, 'REGISTERED_SERVICES:%s' % pkg):
             svc_info = service.split(":")
             try:
                 svc_name, svc_type, svc_port, svc_txt_data = svc_info