sys.platform is missing from the raspberrypi ports.

I started to make a PR, with #define MICROPY_PY_SYS_PLATFORM in mpconfigport.h. However, it seems to require some thought. According to the docs for sys.platform:

For baremetal ports it is an identifier of the chip on a board, e.g. "MicroChip SAMD51". It thus can be used to distinguish one board from another.

(This is exactly what I want it for... to distinguish between boards in the Gameduino driver and assign the appropriate default IO pins.)

So it should be different for the raspberry_pi_pico and adafruit_feather_rp2040 boards. Maybe it should be

"Rpi RP2040"
"Adafruit RP2040"

But unsure. Happy to continue the PR with any given scheme.