Refuse to start a SD print until filament loading is complete

Do not blidnly clear the loading_flag, check for it!

Just disallowing the SD menu while loading is being performed is not
sufficient, since the menu can be entered also by inserting card while
loading is taking place.

This is also nicer in behavior, as we allow to navigate the SD card
while loading.

Firmware/ultralcd.cpp

@@ -6917,7 +6917,8 @@ static bool check_file(const char* filename) {
 
 static void menu_action_sdfile(const char* filename)
 {
-  loading_flag = false;
+  if(loading_flag) return;
+
   char cmd[30];
   char* c;
   bool result = true;