This change moves the calls to mbedtls_sha256_starts() and
mbedtls_sha512_starts() out of the mbedtls_entropy_init() function as
these now have return codes which need to be checked.
Ensure that the entropy self test always fails whenever
MBEDTLS_TEST_NULL_ENTROPY is defined. This is because the option is
meant to be for testing and development purposes rather than production
quality software. Also, this patch enhances the documentation for
mbedtls_entropy_source_self_test() and mbedtls_entropy_self_test().
Instead of polling the hardware entropy source a single time and
comparing the output with itself, the source is polled at least twice
and make sure that the separate outputs are different.
The self test is a quick way to check at startup whether the entropy
sources are functioning correctly. The self test only polls 8 bytes
from the default entropy source and performs the following checks:
- The bytes are not all 0x00 or 0xFF.
- The hardware does not return an error when polled.
- The entropy does not provide data in a patter. Only check pattern
at byte, word and long word sizes.
Update the NV entropy seed before generating any entropy for outside
use. The reason this is triggered here and not in mbedtls_entropy_init(),
is that not all entropy sources mights have been added at that time.
