elog-gen: allow inherits from errors without meta

Previously, elog-gen only support inherits from errors that have meta,
and if we inherit from an error without meta, e.g.
Common.InternalFailure, we get build error like:

    KeyError: 'xyz.openbmc_project.Common.InternalFailure'

This commit makes elog-gen to allow such case, so it is possible to
allow a base error without meta, and inheriting error with additional
error, e.g.

    # In Common:
    - name: NotAllowed  ## No meta

    # In Time:
    - name: NotAllowed
      inherits:
        - xyz.openbmc_project.Common.NotAllowed
      meta:
        - str: "OWNER=%s"
          type: string
        - str: "SYNC_METHOD=%s"
          type: string

Note: the "inherit" here does not mean the the error inherits from
base errors in c++ class, it just makes error "logically" inherits base
errors. And in future commits we could make it "really" inherits base
errors in c++ level.

Tested: Verify phosphor-logging builds correctly with above example
        error interfaces, and service is able to use such errors with
        elog.

Change-Id: I8dccd7112881e3eb77a8f6ec62a532062348d2ef
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/tools/elog-gen.py b/tools/elog-gen.py
index 9169a6e..825fc0f 100755
--- a/tools/elog-gen.py
+++ b/tools/elog-gen.py
@@ -230,8 +230,10 @@
         if 'inherits' in match:
             parents[fullname]  = match['inherits'][0]
 
+        # Put all errors in meta[] even the meta is empty
+        # so that child errors could inherits such error without meta
+        tmp_meta = []
         if 'meta' in match:
-            tmp_meta = []
             # grab all the meta data fields and info
             for i in match['meta']:
                 str_short = i['str'].split('=')[0]
@@ -242,7 +244,7 @@
                 meta_data[str_short]['type'] = get_cpp_type(i['type'])
                 if ('process' in i) and (True == i['process']):
                     metadata_process[str_short] = fullname + "." + str_short
-            meta[fullname] = tmp_meta
+        meta[fullname] = tmp_meta
 
     # Debug
     # for i in errors: