eslintrc: Set "root" to true

Eslint shimmies up directories looking for more eslintrcs. As
make-checkout now clones projects in a subdirectory of cockpit, this can
result in some strange interactions.

https://eslint.org/docs/user-guide/configuring

.eslintrc.json

@@ -1,4 +1,5 @@
 {
+    "root": true,
     "env": {
         "browser": true,
         "es6": true