Make it easier to define MBEDTLS_PARAM_FAILED as assert

Introduce a new configuration option MBEDTLS_CHECK_PARAMS_ASSERT,
which is disabled by default. When this option is enabled,
MBEDTLS_PARAM_FAILED defaults to assert rather than to a call to
mbedtls_param_failed, and <assert.h> is included.

This fixes #2671 (no easy way to make MBEDTLS_PARAM_FAILED assert)
without breaking backward compatibility. With this change,
`config.pl full` runs tests with MBEDTLS_PARAM_FAILED set to assert,
so the tests will fail if a validation check fails, and programs don't
need to provide their own definition of mbedtls_param_failed().

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 764e447..d97f564 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -71,6 +71,8 @@
      Contributed by Peter Kolbus (Garmin).
    * Change wording in the `mbedtls_ssl_conf_max_frag_len()`'s documentation to
      improve clarity. Fixes #2258.
+   * Make it easier to define MBEDTLS_PARAM_FAILED as assert (which config.h
+     suggests). #2671
 
 = mbed TLS 2.17.0 branch released 2019-03-19