Add Adafruit feather and itsybitsy rp2040

[hathach]

Draft PR for discussion only, should be toward rpi/board-headers later on.

This takes discussion from raspberrypi#138 into account. Though the board definition doesn't do much than define UART, LED pin along with Flash size for Feather & ItsyBitsy RP2040. Currently there is no PICO_DEFAULT_ for I2C or SPI, but it still be a good start to have user to blink from pico-sdk on our actual hardware (still need raspberrypi/pico-examples#57)

from RPi document to build with our board

cd pico-examples
mkdir build-feather
cd build-feather
cmake -D"PICO_BOARD=adafruit_feather_rp2040" ..

Then proceed to build normally with generated make.

[ZodiusInfuser]

Hi. A suggestion for you board configs, include a define near the bottom of your files that can be used by code to check what board is being used. For instance the vgaboard.h that is already in the SDK has #define PICO_VGA_BOARD on the third line from the bottom:
https://github.com/raspberrypi/pico-sdk/blob/2d5789eca89658a7f0a01e2d6010c0f254605d72/src/boards/include/boards/vgaboard.h#L102

For our Pimoroni boards we've added #define PIMORONI_TINY2040 and #define PIMORONI_KEYBOW2040

[hathach]

Ah thanks for the tip, it is indeed very handy, I basically mirror the pico.h which doesn't define that. It seems to be quite important, maybe, I should put it in the beginning instead 😃

[hathach]

add PICO_DEFAULT_WS2812_PIN following raspberrypi#188

[hathach]

closed, official PR is submitted to raspberrypi#194