util/nvmutil: more reliable fallback crypto

we assume the fallback will be rare, so now we
make the mix static and keep xoring it, on the
theory that the number of failures on urandom
will be random, and tthat the fallback may only
apply once or twice in thousands of calls.

the time jitter is adjusted; rather than judge
the difference between two points close to each
other in time, we judge tthe randomness in
difference of time elapsed. this mitigates fast
CPUs being very fast and introducing rounding
errors, and also improves performonce on much
slower CPUs

Signed-off-by: Leah Rowe <leah@libreboot.org>

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