Add a lowpower test for the buttons

Resolves #19

firmware/Application/ButtonManager.cpp

@@ -161,7 +161,7 @@ void ButtonManager::irq()
     static BSP::time_t systime;
 
     // TODO: is this call too expensive for an interrupt handler?
-    //BSP::SystemTimer::get_time(systime);
+    BSP::SystemTimer::get_time(systime);
 
     if (!m_instance) {
         return;