Fix some issues with board index generator script

[silvanocerza]

Now generator.py replaces all the necessary parameters in the uploader.command field so the fwuploader will be able to run the upload tool, if that is not done the fwuploader simply lacks the information necessary to replaces those parameters since they're stored in various cores' boards.txt and platform.txt. The fwuploader assumes no core is installed so this had to be done.

I also had to handle a corner case for the Arduino Uno WiFi Rev2 since the upload.pattern in the arduino:megaavr core's platform.txt also assumes the bootloader will be uploaded, I solved this by compiling a Loader Sketch that includes the bootloader and by removing the upload.extra_files parameter.

Also I fixed all paths in the raw_boards.json since they were using my own environment paths, now generator.py can be run by anyone with ease.

[umbynos]

🏁