Support override of memory_bank_config in mbed_app.json (#381)

memory_bank_config can be defined in targets.json or custom_targets.json
per-target, but cannot by application. This enables this override by
adding "target.memory_bank_config" config in target_overrides section of
mbed_app.json as usual.

tools/python/mbed_tools/build/_internal/config/config.py

@@ -41,6 +41,11 @@ def _handle_overrides(self, overrides: Iterable[Override]) -> None:
                 _apply_override(self.data, override)
                 continue
 
+            # Support override of memory_bank_config in mbed_app.json
+            if override.namespace == "target" and override.name == "memory_bank_config":
+                _apply_override(self.data, override)
+                continue
+
             setting = next(
                 filter(
                     lambda x: x.name == override.name and x.namespace == override.namespace,