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/Conditionals.h

 
 #ifndef CONFIGURATION_LCD // Get the LCD defines which are needed first
 
-  #define PIN_EXISTS(PN) (defined(PN##_PIN) && PN##_PIN >= 0)
-
   #define CONFIGURATION_LCD
 
   #if ENABLED(MAKRPANEL)

Marlin/macros.h

 #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

Marlin/ultralcd.cpp

   lcd_implementation_init();
 
   #if ENABLED(NEWPANEL)
-    #if BTN_EN1 > 0
+    #if BUTTON_EXISTS(EN1)
       SET_INPUT(BTN_EN1);
       WRITE(BTN_EN1, HIGH);
     #endif
 
-    #if BTN_EN2 > 0
+    #if BUTTON_EXISTS(EN2)
       SET_INPUT(BTN_EN2);
       WRITE(BTN_EN2, HIGH);
     #endif
 
-    #if BTN_ENC > 0
+    #if BUTTON_EXISTS(ENC)
       SET_INPUT(BTN_ENC);
       WRITE(BTN_ENC, HIGH);
     #endif
     #define encrot3 1
   #endif
 
+  #define GET_BUTTON_STATES(DST) \
+    uint8_t new_##DST = 0; \
+    WRITE(SHIFT_LD, LOW); \
+    WRITE(SHIFT_LD, HIGH); \
+    for (int8_t i = 0; i < 8; i++) { \
+      new_##DST >>= 1; \
+      if (READ(SHIFT_OUT)) SBI(new_##DST, 7); \
+      WRITE(SHIFT_CLK, HIGH); \
+      WRITE(SHIFT_CLK, LOW); \
+    } \
+    DST = ~new_##DST; //invert it, because a pressed switch produces a logical 0
+
+
   /**
    * Read encoder buttons from the hardware registers
    * Warning: This function is called from interrupt context!
   void lcd_buttons_update() {
     #if ENABLED(NEWPANEL)
       uint8_t newbutton = 0;
-      #if BTN_EN1 > 0
-        if (READ(BTN_EN1) == 0) newbutton |= EN_A;
+      #if BUTTON_EXISTS(EN1)
+        if (BUTTON_PRESSED(EN1)) newbutton |= EN_A;
       #endif
-      #if BTN_EN2 > 0
-        if (READ(BTN_EN2) == 0) newbutton |= EN_B;
+      #if BUTTON_EXISTS(EN2)
+        if (BUTTON_PRESSED(EN2)) newbutton |= EN_B;
       #endif
-      #if ENABLED(RIGIDBOT_PANEL) || BTN_ENC > 0
+      #if ENABLED(RIGIDBOT_PANEL) || BUTTON_EXISTS(ENC)
         millis_t now = millis();
       #endif
       #if ENABLED(RIGIDBOT_PANEL)
         if (now > next_button_update_ms) {
-          if (READ(BTN_UP) == 0) {
+          if (BUTTON_PRESSED(UP)) {
             encoderDiff = -1 * (ENCODER_STEPS_PER_MENU_ITEM);
             next_button_update_ms = now + 300;
           }
-          else if (READ(BTN_DWN) == 0) {
+          else if (BUTTON_PRESSED(DWN)) {
             encoderDiff = ENCODER_STEPS_PER_MENU_ITEM;
             next_button_update_ms = now + 300;
           }
-          else if (READ(BTN_LFT) == 0) {
+          else if (BUTTON_PRESSED(LFT)) {
             encoderDiff = -1 * (ENCODER_PULSES_PER_STEP);
             next_button_update_ms = now + 300;
           }
-          else if (READ(BTN_RT) == 0) {
+          else if (BUTTON_PRESSED(RT)) {
             encoderDiff = ENCODER_PULSES_PER_STEP;
             next_button_update_ms = now + 300;
           }
         }
       #endif
-      #if BTN_ENC > 0
-        if (now > next_button_update_ms && READ(BTN_ENC) == 0) newbutton |= EN_C;
+      #if BUTTON_EXISTS(ENC)
+        if (now > next_button_update_ms && BUTTON_PRESSED(ENC)) newbutton |= EN_C;
       #endif
       buttons = newbutton;
       #if ENABLED(LCD_HAS_SLOW_BUTTONS)
         buttons |= slow_buttons;
       #endif
       #if ENABLED(REPRAPWORLD_KEYPAD)
-        // for the reprapworld_keypad
-        uint8_t newbutton_reprapworld_keypad = 0;
-        WRITE(SHIFT_LD, LOW);
-        WRITE(SHIFT_LD, HIGH);
-        for (int8_t i = 0; i < 8; i++) {
-          newbutton_reprapworld_keypad >>= 1;
-          if (READ(SHIFT_OUT)) SBI(newbutton_reprapworld_keypad, 7);
-          WRITE(SHIFT_CLK, HIGH);
-          WRITE(SHIFT_CLK, LOW);
-        }
-        buttons_reprapworld_keypad = ~newbutton_reprapworld_keypad; //invert it, because a pressed switch produces a logical 0
+        GET_BUTTON_STATES(buttons_reprapworld_keypad);
       #endif
-    #else   //read it from the shift register
-      uint8_t newbutton = 0;
-      WRITE(SHIFT_LD, LOW);
-      WRITE(SHIFT_LD, HIGH);
-      unsigned char tmp_buttons = 0;
-      for (int8_t i = 0; i < 8; i++) {
-        newbutton >>= 1;
-        if (READ(SHIFT_OUT)) SBI(newbutton, 7);
-        WRITE(SHIFT_CLK, HIGH);
-        WRITE(SHIFT_CLK, LOW);
-      }
-      buttons = ~newbutton; //invert it, because a pressed switch produces a logical 0
+    #else
+      GET_BUTTON_STATES(buttons);
     #endif //!NEWPANEL
 
     #if ENABLED(REVERSE_MENU_DIRECTION)

Marlin/ultralcd.h

 #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();

Marlin/ultralcd_implementation_hitachi_HD44780.h

   #define EN_B (_BV(BLEN_B)) // The two encoder pins are connected through BTN_EN1 and BTN_EN2
   #define EN_A (_BV(BLEN_A))
 
-  #if defined(BTN_ENC) && BTN_ENC > -1
+  #if BUTTON_EXISTS(ENC)
     // encoder click is directly connected
     #define BLEN_C 2
     #define EN_C (_BV(BLEN_C))
     #define B_DW (BUTTON_DOWN<<B_I2C_BTN_OFFSET)
     #define B_RI (BUTTON_RIGHT<<B_I2C_BTN_OFFSET)
 
-    #if defined(BTN_ENC) && BTN_ENC > -1
+    #if BUTTON_EXISTS(ENC)
       // the pause/stop/restart button is connected to BTN_ENC when used
       #define B_ST (EN_C)                            // Map the pause/stop/resume button into its normalized functional name
       #undef LCD_CLICKED
     #define LCD_HAS_SLOW_BUTTONS
 
   #elif ENABLED(LCD_I2C_PANELOLU2)
-    // encoder click can be read through I2C if not directly connected
-    #if BTN_ENC <= 0
+
+    #if BUTTON_EXISTS(ENC)
+
+      #undef LCD_CLICKED
+      #define LCD_CLICKED (buttons&EN_C)
+
+    #else // Read through I2C if not directly connected to a pin
+
       #define B_I2C_BTN_OFFSET 3 // (the first three bit positions reserved for EN_A, EN_B, EN_C)
 
       #define B_MI (PANELOLU2_ENCODER_C<<B_I2C_BTN_OFFSET) // requires LiquidTWI2 library v1.2.3 or later
 
       // I2C buttons take too long to read inside an interrupt context and so we read them during lcd_update
       #define LCD_HAS_SLOW_BUTTONS
-    #else
-      #undef LCD_CLICKED
-      #define LCD_CLICKED (buttons&EN_C)
+
     #endif
 
   #elif ENABLED(REPRAPWORLD_KEYPAD)