Add a config flag for the global RNG

The global RNG should be provided by the application depending on
the RNG used there. (I.e. TRNG)
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 1a057a4..965efff 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -3683,6 +3683,14 @@
  */
 //#define MBEDTLS_PLATFORM_GMTIME_R_ALT
 
+/**
+ * Uncomment the macro to let Mbed TLS use a platform implementation of
+ * global RNG.
+ *
+ * By default the global RNG function will be a no-op.
+ */
+//#define MBEDTLS_PLATFORM_GLOBAL_RNG
+
 /* \} name SECTION: Customisation configuration options */
 
 /**