fix os.uname() values for rp2350

[dhalbert]

https://forums.adafruit.com/viewtopic.php?t=213955 by blakebr notes that os.uname().sysname and .nodename say rp2040 even for rp2350 boards. Fix this by using MICROPY_HW_MCU_NAME instead. Note that it would be rp2350a and rp2350b.

Note that on some ports, the name is more generic, such as samd21 and samd51 for atmel-samd. But espressif and broadcom use MICROPY_HW_MCU_NAME. In the long run, maybe that is better, since it's more informative, instead of using a generic name. This could be a 10.0 change.

[tannewt]

This is fine. Thanks!