Summary:The local-cli didn't pickup the `rn-cli.config.js` file on Windows because the root of a path is not 1 character long since it is 'C:/' and broke the path operations. This just substrings the root length instead of hard coding 1.
Also fix a lint warning in the file about path concatenation but using string interpolation.
Fixes #5686
**Test plan (required)**
Tested that the `findParentDirectory` function returns the path of `rn-cli.config.js` if present or null if not on both windows and mac.
Closes https://github.com/facebook/react-native/pull/6553
Differential Revision: D3075196
Pulled By: mkonicek
fb-gh-sync-id: a19ab4030ec22d85bef40d7d91de53bc1da072ca
shipit-source-id: a19ab4030ec22d85bef40d7d91de53bc1da072ca