Bug 1611422 - Mark source tree root as root in eslint config r=Standard8

Differential Revision: https://phabricator.services.mozilla.com/D60958

--HG--
extra : moz-landing-system : lando

.eslintrc.js

@@ -46,6 +46,8 @@ const ignorePatterns = [
 
 module.exports = {
   ignorePatterns,
+  // Ignore eslint configurations in parent directories.
+  "root": true,
   // New rules and configurations should generally be added in
   // tools/lint/eslint/eslint-plugin-mozilla/lib/configs/recommended.js to
   // allow external repositories that use the plugin to pick them up as well.