mbedtls: MBEDTLS_ENTROPY_POLL_ZEPHYR default on if MBEDTLS_ENTROPY_C

As long as MBEDTLS_ENTROPY_C is enabled, Mbed TLS needs to
poll some entropy source to gather data that will then be
processed by CTR/HMAC-DRBG modules. This means that in most
of the cases, once MBEDTLS_ENTROPY_C is enabled then also
MBEDTLS_ENTROPY_POLL_ZEPHYR needs to be enabled. This was
done manually until now, as the long list of samples/tests
demonstrate.

This commit solves this dependency by defaulting
MBEDTLS_ENTROPY_POLL_ZEPHYR to on as soon as
MBEDTLS_ENTROPY_C is set. As a consequence, all manual
enablement of MBEDTLS_ENTROPY_POLL_ZEPHYR in samples/tests
are removed.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>