raspberrypi: implement os.urandom

Since the datasheet cast some doubt on the strength of the "rosc_hw->randombit",
I use the SHA256 hash function to create a high quality random seed
from random values of uncertain entropy, as well as to generate a sequence
of random values from that seed using SHA256 as a cryptographically-secure
random number generator.

In practice, it produces over 100kB/s of random data which does not
have any gross problems according to _PractRand_.

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