Remove SAVE_ON_FLASH=1 for arminarm board

Sorry to bother you with another pull request. The SAVE_ON_FLASH=1 slipped in the Makefile for ARMinARM here: https://github.com/espruino/Espruino/pull/431

I totally misinterpreted that. It would be a shame to miss out on the functionality of jswrap_waveform.c for example. :)

Thanks again!