Template overlays don't work

I tried to build from an overlay template as specified in this repo's README.

config:
```
{
"overlays": [
"overlays/example"
],
"repositories": [
"default",
"example"
],
"build": {
"template": "example"
}
}
```

where the 'example' template files were in `src/overlays/example/templates?dist`. Looks like the `getTemplateFile()` function in `os-js-build`'s `webpack.js` has a hardcoded root path ?

Start with the README overlay example and the getTemplateFile() function in os-js-build's webpack.js. Trace how it resolves files under src/overlays/example/templates?dist, then verify that building with the supplied configuration successfully finds the example template.

Written by the indexing model from the issue text.