`purge` rule is meant to clean the device, so I don't think it is relevant to clean settings here.
Don't you just want to do `make clean` that remove the whole profile folder?

Otherwise, if it is important to clean the profile generation each time this file change, we can:
1) Rename the rule `profile/settings.json` to `settings`
2) Make it depend on `profile/settings.json`
3) Remove `profile/settings.json` each time this rule is executed
```
settings: profile/settings.json
  rm -f profile/settings.json
  ...
```