config.pl full: exclude MBEDTLS_ENTROPY_FORCE_SHA256

This is a variant toggle, not an extra feature, so it should be tested
separately.

We test most of the effect of MBEDTLS_ENTROPY_FORCE_SHA256 (namely,
using SHA-256 in the entropy module) when we test the library with the
SHA512 module disabled (which we do at least via depends-hashes.pl).
This commit removes testing of the MBEDTLS_ENTROPY_FORCE_SHA256 option
itself, which should be added separately.
diff --git a/scripts/config.py b/scripts/config.py
index db2661c..cb0e1c5 100755
--- a/scripts/config.py
+++ b/scripts/config.py
@@ -168,6 +168,7 @@
             'MBEDTLS_DEPRECATED_REMOVED',
             'MBEDTLS_ECDH_VARIANT_EVEREST_ENABLED',
             'MBEDTLS_ECP_RESTARTABLE',
+            'MBEDTLS_ENTROPY_FORCE_SHA256', # Variant toggle, tested separately
             'MBEDTLS_HAVE_SSE2',
             'MBEDTLS_MEMORY_BACKTRACE',
             'MBEDTLS_MEMORY_BUFFER_ALLOC_C',