[ion] Add TODO on implementations of usleep msleep

ion/src/device/device.cpp

@@ -19,6 +19,10 @@ extern "C" {
 
 // Public Ion methods
 
+/* TODO: The delay methods 'msleep' and 'usleep' are currently dependent on the
+ * optimizations chosen by the compiler. To prevent that and to gain in
+ * precision, we could use the controller cycle counter (Systick). */
+
 void Ion::msleep(long ms) {
   for (volatile long i=0; i<8852*ms; i++) {
       __asm volatile("nop");