Why not just sample a noise diode, or thermal noise from a highly amplified resistor? Is the process of amplification (i.e. periodic power supply noise) enough to influence the randomness?
NeuG has some low quality sources of entropy, and also provides analog input for the user to supply with whatever they wish. It does not address the design of a good white noise generator, unfortunately.
you'd still have to feed that through something to get it into a PC. the STM32 is about the cheapest way of doing that. and it turns out it comes with some ADCs that are so noisy they're good sources of randomness themselves.