Add button tests

This is implemented by connecting the DTR pin of the serial device to the BTN_UP pin of the watch.

Also, make it possible to flash different applications with the Makefile.

Resolves #4

firmware/Application/ButtonManager.h

@@ -54,9 +54,9 @@ public:
     static ButtonManager *m_instance;
 
     enum Button {
-        UP = 0,
+        DOWN = 0,
         MID,
-        DOWN,
+        UP,
         Count
     };