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.

include/mbedtls/entropy_poll.h

@@ -43,6 +43,14 @@ extern "C" {
 #define MBEDTLS_ENTROPY_MIN_HARDCLOCK     4     /**< Minimum for mbedtls_timing_hardclock()        */
 #define MBEDTLS_ENTROPY_MIN_HARDWARE     32     /**< Minimum for the hardware source */
 
+/**
+ * \brief           Entropy poll callback that provides 0 entropy.
+ */
+#if defined(MBEDTLS_TEST_WO_ENTROPY)
+    int mbedtls_zero_entropy_poll( void *data,
+                                unsigned char *output, size_t len, size_t *olen );
+#endif
+
 #if !defined(MBEDTLS_NO_PLATFORM_ENTROPY)
 /**
  * \brief           Platform-specific entropy poll callback