WebApp should prompt user to install firmware if needed

[John Maloney]

If the user tries to connect to a micro:bit that doesn’t have the MicroBlocks firmware installed, the serial port will open but the board won’t respond to ping message. In that case, MicroBlocks should prompt the user to install the MicroBlocks firmware. (Note that the Web App can’t detect the type of board, but it can install firmware on micro:bit and AdaFruit boards after the user selects the board type from a menu.)