We want to support builds where there are no platform entropy
sources (`MBEDTLS_NO_PLATFORM_ENTROPY` enabled), and no custom entropy
sources (`MBEDTLS_ENTROPY_HARDWARE_ALT` disabled), but
`mbedtls_entropy_init()` sets up a working entropy without needing to add
sources manually with `mbedtls_entropy_add_source()`
(`MBEDTLS_NO_DEFAULT_ENTROPY_SOURCES` disabled) thanks to a nonvolatile seed
file injected outside the library's control (`MBEDTLS_ENTROPY_NV_SEED`
enabled).
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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