extmod/modmachine: Provide common Python bindings for machine.idle().

And use it in all ports.  The ports are unchanged, except esp8266 which now
just returns None from this function instead of the time elapsed (to match
other ports), and qemu-arm which gains this function.

Signed-off-by: Damien George <damien@micropython.org>

ports/qemu-arm/modmachine.c

@@ -32,3 +32,7 @@
 
 // This variable is needed for machine.soft_reset(), but the variable is otherwise unused.
 int pyexec_system_exit = 0;
+
+STATIC void mp_machine_idle(void) {
+    // Do nothing.
+}