bootstrapPath var missing for mosaic

On Plone 5.2.1 this var is missing (not sure if on older versions too). Adding it in the web-ui as LESS-Variable fixed it. But i cant find where it should get added / get lost?

```
ERROR in ./src/lakof.theme/logged-in.less (./node_modules/css-loader/dist/cjs.js??ref--6-1!./node_modules/less-loader/dist/cjs.js??ref--6-2!./src/lakof.theme/logged-in.less)
Module build failed (from ./node_modules/less-loader/dist/cjs.js):

@import (reference) "@{barcelonetaPath}/less/variables.plone.less";
@import (reference) "@{bootstrapPath}/less/bootstrap.less";
^
Variable @bootstrapPath is undefined
```

Start with src/lakof.theme/logged-in.less and the webpack/LESS setup that supplies barcelonetaPath and bootstrapPath. Reproduce the build error, then trace where bootstrapPath is expected to be defined or lost. Done means the Plone 5.2.1 build completes without manually adding the variable in the web UI.

Written by the indexing model from the issue text.