boards/avr/atmega: Added Elegoo Mega2560r3 board support

[linguini1]

Summary

This PR adds support for a NSH configuration on the Elegoo ATMega2560 Rev3 board.

Impact

Users with this board will be able to use NuttX on it.

This impacts the build system by adding a new board (some new configuration options added), the documentation system and adds a new board under the avr/atmega grouping.

Testing

Documentation was tested with local build and verification of render in browser.

autoled implementation was tested by enabling the subsystem and calling board_autoled_on(LED_STARTED) in the atmega_boardinitialize() function, verifying that the LED turned on.

The NSH application could not be tested as of right now. It appears the UART driver for the ATMega2560 is not working correctly. No data appears on USART0 after booting, even with syslog enabled. When I manually call avr_lowputc in the atmega_boardinitialize() function, the TX LED on the board gets stuck on.

Since this board is a "clone" of the Arduino ATMega2560 board, I also flashed the hello configuration from that board and experienced the same lack of data on USART0. If there is anyone with an ATMega2560 based board who might be able to confirm this issue, please let me know! I'm linking this to #16444.

[linguini1]

Is there anyone with an ATMega2560-based board that can help me test the serial console/boot implementation?

I am able to get some up_putc output to appear, but I experience weird crashes and the board cannot launch into application code (see #16444). I'm not able to get a working build of arduino-mega2560:nsh as far back as release 11.0 (further back and I run into compiler errors so I've stopped for now).

[linguini1]

Don't think the build error is the cause of this PR.

[simbit18]

this error is not about this PR

Job Linux (risc-v-06)

curl: (16) Error in the HTTP2 framing layer
make[1]: *** [fdt/Make.defs:30: dtc] Error 16
make[1]: Target 'context' not remade because of errors.
make: *** [tools/Unix.mk:458: libs/libc/.context] Error 2
make: Target 'all' not remade because of errors.
/github/workspace/sources/nuttx/tools/testbuild.sh: line 385: /github/workspace/sources/nuttx/../nuttx/nuttx.manifest: No such file or directory