Add entropy safety switch.

Add a switch that turns entropy collecting off entirely, but enables
mbed TLS to run in an entirely unsafe mode. Enables to test mbed TLS
on platforms that don't have their entropy sources integrated yet.

library/entropy.c

@@ -73,6 +73,11 @@ void mbedtls_entropy_init( mbedtls_entropy_context *ctx )
     mbedtls_havege_init( &ctx->havege_data );
 #endif
 
+#if defined(MBEDTLS_TEST_WO_ENTROPY)
+    mbedtls_entropy_add_source( ctx, mbedtls_zero_entropy_poll, NULL,
+                                1, MBEDTLS_ENTROPY_SOURCE_STRONG );
+#endif
+
 #if !defined(MBEDTLS_NO_DEFAULT_ENTROPY_SOURCES)
 #if !defined(MBEDTLS_NO_PLATFORM_ENTROPY)
     mbedtls_entropy_add_source( ctx, mbedtls_platform_entropy_poll, NULL,