Enable warnings on nRF5x examples

Now all warnings are disabled on SDK files, and enabled on our
application's files.

Fixed a couple of things in nRF52 which were unnoticed because all
warnings were disabled there from the beginning.

PUBLISHED_FROM=a33ab22d0ce85efea364b80478986b88f4b0a3a2

examples/nRF52/http/main.c

@@ -85,7 +85,6 @@ int main(void)
 
     for (;;) {  // Start infinite event loop
       bleconfig_poll();
-      sys_check_timeouts();
       mg_mgr_poll(&mgr, 0);
     }