Add "override" to compiler parser options

This is required to emulate the behavior of more complex flags, where:
- There is a default value;
- The default value must be overridden if the flag is provided; and
- The flag can be supplied more than once, and extends a list.

Signed-off-by: John Pennycook <john.pennycook@intel.com>

Author: Pennycook

codebasin/schema/cbiconfig.schema

@@ -60,6 +60,9 @@
                           }
                         }
                       ]
+                    },
+                    "override": {
+                        "type": "boolean"
                     }
                   },
                   "additionalProperties": false

tests/schema/test_schema.py

@@ -121,6 +121,7 @@ def test_cbiconfig_compilers(self):
             dest = "passes"
             pattern = "*"
             format = "$value"
+            override = true
         """,
         )