Crypto: Highlight unsafe NULL entropy more explicitly

The MBEDTLS_TEST_NULL_ENTROPY is only to used for test purposes and
production platforms must use a hardware entropy source for stronger
cryptographic security. Although there is a build warning message
from MbedCrypto regarding this, this patch makes this more explicit
by adding a runtime message and clarifying the integration guide.

Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: I773a3c509dafdd5ffcd46ae5227acbc84aa81129
2 files changed