Rename the 'no entropy' feature to MBEDTLS_TEST_NULL_ENTROPY

Following review and for clarity, changed the name of the feature to 'null
entropy'.
diff --git a/scripts/config.pl b/scripts/config.pl
index ea77821..84ec38e 100755
--- a/scripts/config.pl
+++ b/scripts/config.pl
@@ -18,7 +18,7 @@
 #
 # Things that shouldn't be enabled with "full".
 #
-#   MBEDTLS_TEST_WO_ENTROPY
+#   MBEDTLS_TEST_NULL_ENTROPY
 #   MBEDTLS_DEPRECATED_REMOVED
 #   MBEDTLS_HAVE_SSE2
 #   MBEDTLS_PLATFORM_NO_STD_FUNCTIONS
@@ -70,7 +70,7 @@
 EOU
 
 my @excluded = qw(
-MBEDTLS_TEST_WO_ENTROPY
+MBEDTLS_TEST_NULL_ENTROPY
 MBEDTLS_DEPRECATED_REMOVED
 MBEDTLS_HAVE_SSE2
 MBEDTLS_PLATFORM_NO_STD_FUNCTIONS