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/lib/meson.build b/lib/meson.build
index 8446767..2679548 100644
--- a/lib/meson.build
+++ b/lib/meson.build
@@ -1,6 +1,19 @@
 phosphor_logging_includes = include_directories('include')
 
-phosphor_logging_gen = []
+event_filter_json = get_option('event-filter')
+if event_filter_json == ''
+    event_filter_json = default_eventfilter_json
+endif
+
+lg2_eventfilter_cpp_gen = custom_target(
+    'lg2_eventfilter.cpp',
+    input: [eventfilter_gen, template_eventfilter_gen, event_filter_json],
+    output: 'lg2_eventfilter.cpp',
+    command: [python_prog, '@INPUT0@', '@INPUT2@'],
+    capture: true,
+)
+
+phosphor_logging_gen = [lg2_eventfilter_cpp_gen]
 
 subdir('include/phosphor-logging')
 
@@ -43,4 +56,3 @@
     sources: phosphor_logging_gen,
     dependencies: phosphor_logging_deps,
 )
-