Add support for offline bootloader installation (other means of Internet access still needed)

Add support for offline bootloader installation, so the installation can still be done if there is another means of Internet access, such as another computer or even (perhaps) a mobile phone. The connection will ideally not have to be to the computer running wxfixboot, and files will have to be downloaded via another pc / mobile phone, but I'm yet to figure out how to do this.

Will not be added as WxFixBoot is retired.