tools: add gen-eventfilter.py

Generate event filtering code for lg2::commit from an input json
file, so that system integrators can create default filtering rules
to omit undesired events and errors from their systems.

Tested:

Used `log-create` to create an event and observed it.  Modified the
default event filter as follows and saw log was not created.

```
@@ -3,6 +3,9 @@
         "default": "allowed"
     },
     "errors": {
-        "default": "allowed"
+        "default": "allowed",
+        "ids": [
+            "xyz.openbmc_project.State.SMC.SMCFailed"
+        ]
     }
 }
```

```
$ ./builddir/log-create xyz.openbmc_project.State.SMC.SMCFailed --json '{ "IDENTIFIER": "/xyz/openbmc_project/inventory/SomeSMC", "FAILURE_TYPE": "Timeout for the SMC" }'
```

Change-Id: Ib6041481075758b994bb27a816dbf5e9f26c2841
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.options b/meson.options
index 6f8bdb4..5b3ef17 100644
--- a/meson.options
+++ b/meson.options
@@ -48,3 +48,9 @@
     choices: ['dbus', 'journal', 'both'],
     value: 'both',
 )
+
+option(
+    'event-filter',
+    type: 'string',
+    description: 'Path to the event filter JSON file.',
+)