Grunt config is overwritten on Magento updates via composer

[mombrea]

This issue occurs on composer upgrades of Magento CE 2.x.x

Latest instance occurs in upgrade to Magento CE version 2.1.2

Steps to reproduce

1. Configure your theme for Grunt in /dev/tools/grunt/config/themes.js
2. Update Magento using composer

Expected result

1. Magento should update but not overwrite configuration files

Actual result

1. /dev/tools/grunt/config/themes.js is overwritten with the default version

[veloraven]

@mombrea please identify which version of Magento you are running.

[mombrea]

@veloraven Updated to indicate Magento 2 CE through version 2.1.2

I have experienced this also several times, was not sure if it was a bug or not.

[tkacheva]

Internal issue MAGETWO-61147

[Igloczek]

@omiroshnichenko described solution in #8910

• Copy grunt-config.json.sample to grunt-config.json.
• Copy dev/tools/grunt/configs/themes.js to dev/tools/grunt/configs/local-themes.js
• Specify your themes config in dev/tools/grunt/configs/local-themes.js

[okorshenko]

@mombrea the issue has been fixed in develop branch. Thank you for reporting the issue
@Igloczek thank you for the provided fix