Apply standard pin test to buttons

This is the easiest way to make button pin testing consistent without
renaming all the button pins. Just make a macro especially for testing
if button pins are set, since they are named consistently in the pins
files.

Marlin/ultralcd.h

@@ -24,9 +24,14 @@
 #define ULTRALCD_H
 
 #include "Marlin.h"
+
 #if ENABLED(ULTRA_LCD)
+
   #include "buzzer.h"
 
+  #define BUTTON_EXISTS(BN) (defined(BTN_## BN) && BTN_## BN >= 0)
+  #define BUTTON_PRESSED(BN) !READ(BTN_## BN)
+
   int lcd_strlen(const char* s);
   int lcd_strlen_P(const char* s);
   void lcd_update();