When exporting to Simplicity Studio there is no mbed_config.h in the export (or it's not generated by the build tool, either way). Removing the -include flag from the build flags create a successful build. As far as I can see mbed_config is not used for any of the EFM32 boards, so perhaps we can just remove that flag from the exporter?
See also https://forums.mbed.com/t/mbed-online-compiler-export-problem/1859