Fix ArduinoISP sketch for Uno WiFi Rev2

[per1234]

When using bit banged SPI, which the sketch did when compiled for any architecture other than AVR, a SPISettings class was declared by the sketch. At the time the sketch was written, it was reasonable to expect this would not cause a name collision, since SPI.h is not #included when doing bit banged SPI. However, since then a SPISettings class has been declared in ArduinoCore-API's HardwareSPI.h, causing the ArduinoISP sketch to not compile for any board whose core uses ArduinoCoreAPI (currently Arduino Mega AVR Boards, "Arduino nRF528x Boards (Mbed OS)", and "Arduino Mbed OS Boards (nRF52840 / STM32H747)"):

  /github/workspace/examples/11.ArduinoISP/ArduinoISP/ArduinoISP.ino:191:27: error: reference to 'SPISettings' is ambiguous
       void beginTransaction(SPISettings settings) {
                             ^~~~~~~~~~~
  /github/workspace/examples/11.ArduinoISP/ArduinoISP/ArduinoISP.ino:167:7: note: candidates are: class SPISettings
   class SPISettings {
         ^~~~~~~~~~~
  In file included from /github/home/.arduino15/packages/arduino/hardware/megaavr/1.8.6/cores/arduino/api/ArduinoAPI.h:31:0,
                   from /github/home/.arduino15/packages/arduino/hardware/megaavr/1.8.6/cores/arduino/Arduino.h:23,
                   from /github/workspace/examples/11.ArduinoISP/ArduinoISP/ArduinoISP.ino:39:
  /github/home/.arduino15/packages/arduino/hardware/megaavr/1.8.6/cores/arduino/api/HardwareSPI.h:37:7: note:                 class arduino::SPISettings
   class SPISettings {
         ^~~~~~~~~~~

Although it's easy enough to fix the name collision (#3), it seemed a better idea to simply use hardware SPI when compiling for the megaAVR architecture.

---

After this change, I was able to use the Uno WiFi Rev2 as an "Arduino as ISP" programmer with "Burn Bootloader" and "Upload Using Programmer". It is necessary to use the Tools > Programmer > Arduino as ISP (ATmega32U4).

---

Unfortunately, I found that, although the sketch compiles, I still can't use the Nano Every as an "Arduino as ISP" programmer. It is able to read the chip signature and read and write fuses, but there are avrdude: stk500_recv(): programmer is not responding errors while writing the program to flash.

I found that the same issue occurs if I use the Nano Every and Nano 33 BLE with the bit banged SPI code (after fixing the name collision), while the Uno WiFi Rev2 works fine with the bit banged SPI.

[ubidefeo]

@per1234
I'm fairly sure it's a timing issue.
For the Nano Every and WiFi Rev2, could this be related to the 4809's AVR registry emulation?
For the 33 BLE we'll have to drag Martino in, I guess

[per1234]

I think it must be. I tried it both with and without the ATmega328 emulation enabled for both boards and I got the same result (Uno WiFi Rev2 always works, Nano Every always fails).

When I use a SAMD board, either with the release version of the platform, or a beta version that has ArduinoCore-API integration, it works.

[per1234]

The CI didn't run on the PR because the repo is private, but you can see the jobs here:
https://github.com/per1234/arduino-examples/actions/runs/239053006