[dv regr tool] Added 'overrides' directive

- You can now specify overrides for default sim key values using the
overrides keyword.
- Example:
```hjson

  // In 'common' cfg, specify a default attribute
  foo: bar

  // In DUT specific cfg, provide an override for it
  overrides: [
    {
      name: foo
      value: baz
    }
  ]
```

- This us useful to set a project-default attribute and use overrides if
an IP needs a custom version of it.
- This is needed to setup the hjson cfg for the chip level where the
coverage collection model is different.

Signed-off-by: Srikrishna Iyer <sriyer@google.com>
diff --git a/util/dvsim/SimCfg.py b/util/dvsim/SimCfg.py
index 87db098..13f80ff 100644
--- a/util/dvsim/SimCfg.py
+++ b/util/dvsim/SimCfg.py
@@ -94,7 +94,6 @@
         self.flist_file = ""
         self.run_cmd = ""
         self.dump_file = ""
-        self.exports = []
 
         # Generated data structures
         self.links = {}
@@ -112,6 +111,9 @@
         if self.build_unique:
             self.build_dir += "_" + self.timestamp
 
+        # Process overrides before substituting the wildcards.
+        self._process_overrides()
+
         # Make substitutions, while ignoring the following wildcards
         # TODO: Find a way to set these in sim cfg instead
         ignored_wildcards = [