Fixed fuse flashing in Nano Every configuration

[d3lta-v]

I noticed an issue with the current release of MegaCoreX while developing my own board with the following design choices:

• Uses a SAMD11 for a combined UART+UPDI to USB interface, with JTAG2UPDI and the need for 1200bps touch (identical to the Arduino Nano)
• Uses an ATmega4809
• Uses Arduino Uno Wifi pinout and is attached to an onboard ESP32 Wi-Fi chipset

I modified the core in my own internal builds by duplicating the Nano Every's board definition and changed the build.variant to uno-wifi for that specific pinout, while still retaining the other properties for a SAMD11 based programmer (1200bps touch, JTAG2UPDI).

4809.menu.pinout.sstuinoii=SSTuino II
4809.menu.pinout.sstuinoii.build.variant=uno-wifi
4809.menu.pinout.sstuinoii.upload.tool=avrdude_nanoevery
4809.menu.pinout.sstuinoii.upload.use_1200bps_touch=true
4809.menu.pinout.sstuinoii.upload.protocol=jtag2updi
4809.menu.pinout.sstuinoii.program.extra_params=-P{serial.port} -e
4809.menu.pinout.sstuinoii.build.compat=

I started running into UART issues when I changed the board frequency from 20MHz to 16MHz (and by extension, all clock speeds that depend on the 16MHz oscillator). I tried to dump the fuses from the ATmega4809 and found out that the fuses were not burnt, meaning that while F_CPU was altered, the relevant fuse was never changed to reflect the correct frequency.

I dug around further and found that the {bootloader.fuse0} in platform.txt (and by extension, the other fuse and lock bits) were all blank, as tools.avrdude_nanoevery never had definitions for those variables. This commit fixes the issue by adding those variables, although there might be a more elegant solution than duplicating tools.avrdude.bootloader.<insert fuse here> into tools.avrdude_nanoevery.bootloader.<insert fuse here>. If others have more insights on this, please chime in as this is the first time I've modified platform.txt like this :)

[MCUdude]

Thanks for the detailed breakdown of the issue. I'll give it a try on my own Nano Every and then merge if everything seems OK!

[MCUdude]

Thank you for your contribution!