Changes to allow the entropy tests to work for configurations without an
entropy seed file (MBEDTLS_ENTROPY_NV_SEED), and with no entropy sources
configured (MBEDTLS_TEST_NULL_ENTROPY).
A standard 'test' that writes a seed file is added so that regular tests
still can succeed. This is in lieu of a 'SUITE_PRE_CODE' kind of
arrangement where a suite can run code before (and after) all other code
runs.
A test is added that checks if we can read and write the standard NV
seed file
A test is added that actually checks if the entropy and seed file values
that are the result of just using the NV seed are the same as the manual
calculation.
