Force HostName property to be read-only per the schema

The ManagerNetworkProtocol schema defines the HostName entry to be
read-only. Change the doPatch code to prevent updating the hostname
attribute.

The DMTF redfish/v1/Managers/bmc/NetworkProtocol is a read-only
location.

The DMTF approved location for changing the HostName is:
redfish/v1/Managers/bmc/EthernetInterfaces/<str>

This change does not impact phosphor-webui, as it uses D-Bus to
perform all of its work.
This change does not impact webui-vue, as it is using the DMTF
approved API.

This commit deprecates allowing Read/Write access to the Hostname in
the ManagersNetworkProtocol URI. To reduce the impact to Redfish
clients that rely upon Read/Write access a Meson compile time flag has
been added to allow Read/Write access to be restored. The Meson build
flag, redfish-allow-deprecated-hostname-patch, can be enabled to
restore Read/Write access. The Meson build flag is slated to be
removed in Q4 2021 enforcing the read-only state.

Tested:
Explicitly PATCH'd HostName to confirm it cannot be modified.
Enabled the HostName feature, and confirmed the HostName accepted a
PATCH command.
Ran Redfish_Service_Validator (deprecated, and re-enabled).

Change-Id: If7f2148d8bbb8a7b420c4abde086272c4320977a
Signed-off-by: Johnathan Mantey <johnathanx.mantey@intel.com>
diff --git a/meson.build b/meson.build
index 1bb7636..66a066b 100644
--- a/meson.build
+++ b/meson.build
@@ -206,6 +206,16 @@
               },section : 'Enabled Features')
     endif
 
+    if( get_option('redfish-allow-deprecated-hostname-patch').enabled())
+     add_project_arguments([
+       '-DBMCWEB_ALLOW_DEPRECATED_HOSTNAME_PATCH'
+       ],
+      language : 'cpp')
+
+      summary({'hostname-patch' :'-DBMCWEB_ALLOW_DEPRECATED_HOSTNAME_PATCH'
+              },section : 'Enabled Features')
+    endif
+
   endif
 endif