Semi-unrelated question: How secure are the hardware random number generators on say, a raspberry pi. Would these sort of things be cryptographically secure?
I think those built-in RNGs should be treated like RDRAND on x86, you may mix them into the pool along with all the other sources of entropy your platform supplies, and then generate cryptographic streams seeded out of the pool. Using them alone might turn out to be a bad idea in the long run.
Would /dev/random be the combined entropy?
No, that would be the output of some hash/crypto stream(s) that in turn gets seeded by all the entropy sources the machine can find, mixed together.