updated picoTCP to 1.4.0, lowered build optimization levels to -O2, improved selftest

ext/picotcp/test/examples/noop.c

@@ -0,0 +1,11 @@
+/* NOOP */
+#include <pico_stack.h>
+void app_noop(void)
+{
+    while(1) {
+        pico_stack_tick();
+        usleep(2000);
+    }
+}
+
+/* END NOOP */