core: fail build when CFG_INSECURE=n used with unsafe RNG seed

A production build needs a proper RNG seed. Fail the build when
CFG_INSECURE=n is used in conjunction with the default weak stub
implementation of plat_rng_init().

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>

core/tee/tee_cryp_utl.c

@@ -200,7 +200,14 @@ void __plat_rng_init(void)
 	}
 }
 
+/*
+ * Override this in your platform code. This default implementation only seeds
+ * the random number generator from an easily predictable timestamp value or a
+ * constant value. It is not suitable for a secure environment.
+ */
+#ifdef CFG_INSECURE
 void plat_rng_init(void) __weak __alias("__plat_rng_init");
+#endif
 
 static TEE_Result tee_cryp_init(void)
 {