Raspberry Pi is a difficult target. It is not Open Hardware, and there
is a bunch of critical closed-source system software. The Broadcom
SOCs are not fully documented.
All true, but there are a few documents floating around like:
https://datasheets.raspberrypi.com/bcm2711/bcm2711-peripherals.pdf
Support for the RPi zero with the BCM2708 was started and was partially
functional, but was never finished:
*
https://bitbucket.org/patacongo/obsoleted/src/master/nuttx/arch/arm/src/bcm2708/
* https://bitbucket.org/patacongo/obsoleted/src/master/nuttx/configs/pizero/
The RP2040 however is a complete different thing. There is no Broadcom
involvement, and the chip seems to be fully documented, and there is a
NuttX port.
Beaglebone Black is another supported SBC option. Also the older
pcDuino SBC.
