.. all the necessary steps to transform it into a padded, checksummed
file are now done by the build system.
Since it is assigned by "?=", it _should_ be the case that individual
builds can override it.
I did not "test" this per se, but it gives the same content and checksum
(except for the identifying comment with a path) as #4302.
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_.
The RP2040 is new microcontroller from Raspberry Pi that features
two Cortex M0s and eight PIO state machines that are good for
crunching lots of data. It has 264k RAM and a built in UF2
bootloader too.
Datasheet: https://pico.raspberrypi.org/files/rp2040_datasheet.pdf
