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/macros.h

@@ -55,4 +55,6 @@
 #define NUMERIC_SIGNED(a) (NUMERIC(a) || (a) == '-')
 #define COUNT(a) (sizeof(a)/sizeof(*a))
 
+#define PIN_EXISTS(PN) (defined(PN ##_PIN) && PN ##_PIN >= 0)
+
 #endif //__MACROS_H