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/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 63f93ec..a95af6c 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -130,15 +130,14 @@
 #error "MBEDTLS_ENTROPY_FORCE_SHA256 defined, but not all prerequisites"
 #endif
 
-#if defined(MBEDTLS_TEST_WO_ENTROPY)
-#warning "MBEDTLS_TEST_WO_ENTROPY defined, this build provides no security!"
-#if !defined(MBEDTLS_ENTROPY_C) || !defined(MBEDTLS_NO_DEFAULT_ENTROPY_SOURCES)
-#error "MBEDTLS_TEST_WO_ENTROPY defined, but not all prerequisites"
+#if defined(MBEDTLS_TEST_NULL_ENTROPY) && \
+    ( !defined(MBEDTLS_ENTROPY_C) || !defined(MBEDTLS_NO_DEFAULT_ENTROPY_SOURCES) )
+#error "MBEDTLS_TEST_NULL_ENTROPY defined, but not all prerequisites"
 #endif
-#if defined(MBEDTLS_ENTROPY_NV_SEED) || defined(MBEDTLS_ENTROPY_HARDWARE_ALT) || \
-    defined(MBEDTLS_HAVEGE_C)
-#error "MBEDTLS_TEST_WO_ENTROPY defined, but entropy sources too"
-#endif
+#if defined(MBEDTLS_TEST_NULL_ENTROPY) && \
+     ( defined(MBEDTLS_ENTROPY_NV_SEED) || defined(MBEDTLS_ENTROPY_HARDWARE_ALT) || \
+    defined(MBEDTLS_HAVEGE_C) )
+#error "MBEDTLS_TEST_NULL_ENTROPY defined, but entropy sources too"
 #endif
 
 #if defined(MBEDTLS_GCM_C) && (                                        \