Created by: cliff76
On certain platforms (for eg. Tizen TVs) we need the ability to enable a specific type of sourcemap (eval-source-map). True/false is too limiting.
I verified this minor change by running yarn build with different values for the GENERATE_SOURCEMAP environment variable then inspecting the resulting main..js under build/static/js/ Setting the env var to false preserves existing behavior. Setting it to other allowed sourcemap values correctly changes the resulting output JS bundle.