Add Einstart-S support (#11219)

Indeed the HAL does not need to be mucked around with to expose ATmega2560 pins not available as numbers on the MEGA board, I'll need to update the wiki with that tidbit and a reference to the pin-mapping comment in `fastio_1280.h`.

Marlin/src/pins/pins.h

@@ -183,6 +183,8 @@
   #include "pins_GT2560_REV_A.h"      // ATmega1280, ATmega2560                     env:megaatmega1280 env:megaatmega2560
 #elif MB(GT2560_REV_A_PLUS)
   #include "pins_GT2560_REV_A_PLUS.h" // ATmega1280, ATmega2560                     env:megaatmega1280 env:megaatmega2560
+#elif MB(EINSTART_S)
+  #include "pins_EINSTART-S.h"        // ATmega1280, ATmega2560                     env:megaatmega1280 env:megaatmega2560
 
 //
 // ATmega1281, ATmega2561

Marlin/src/pins/pins_EINSTART-S.h

@@ -0,0 +1,122 @@
+/**
+ * Marlin 3D Printer Firmware
+ * Copyright (C) 2016 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
+ *
+ * Based on Sprinter and grbl.
+ * Copyright (C) 2011 Camiel Gubbels / Erik van der Zalm
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+/**
+ * Einstart S pin assignments
+ * PCB Silkscreen: 3DPrinterCon_v3.5
+ */
+
+#if ENABLED(TARGET_LPC1768)
+  #error "Oops!  Set MOTHERBOARD to an LPC1768-based board when building for LPC1768."
+#elif defined(__STM32F1__)
+  #error "Oops!  Set MOTHERBOARD to an STM32F1-based board when building for STM32F1."
+#endif
+
+#if DISABLED(IS_RAMPS_SMART) && DISABLED(IS_RAMPS_DUO) && DISABLED(IS_RAMPS4DUE) && DISABLED(TARGET_LPC1768)
+  #if !defined(__AVR_ATmega1280__) && !defined(__AVR_ATmega2560__)
+    #error "Oops!  Make sure you have 'Arduino Mega' selected from the 'Tools -> Boards' menu."
+  #endif
+#endif
+
+#ifndef BOARD_NAME
+  #define BOARD_NAME "EINSTART_S"
+#endif
+
+//
+// Limit Switches
+//
+#define X_STOP_PIN         44   // 2560 PIN 40
+#define Y_STOP_PIN         43   // 2560 PIN 41 
+#define Z_STOP_PIN         42   // 2560 PIN 42
+
+//
+// Steppers
+//
+#define X_STEP_PIN         76   // 2560 PIN 68
+#define X_DIR_PIN          75   // 2560 PIN 67
+#define X_ENABLE_PIN       73   // 2560 PIN 66
+
+#define Y_STEP_PIN         31   // 2560 PIN 59
+#define Y_DIR_PIN          32   // 2560 PIN 58
+#define Y_ENABLE_PIN       72   // 2560 PIN 65
+
+#define Z_STEP_PIN         34   // 2560 PIN 56
+#define Z_DIR_PIN          35   // 2560 PIN 55
+#define Z_ENABLE_PIN       33   // 2560 PIN 57
+
+#define E0_STEP_PIN        36   // 2560 PIN 54
+#define E0_DIR_PIN         37   // 2560 PIN 53
+#define E0_ENABLE_PIN      30   // 2560 PIN 60
+
+//
+// Temperature Sensors
+//
+#define TEMP_0_PIN          0   // Analog Input
+#define TEMP_BED_PIN        1   // Analog Input
+
+//
+// Heaters / Fans
+//
+#define HEATER_0_PIN       83   // 2560 PIN 49
+#define HEATER_BED_PIN     38   // 2560 PIN 50
+
+#define FAN_PIN            82   // 2560 PIN 48
+
+//
+// Misc. Functions
+//
+#define SDSS               53   // 2560 PIN 19
+#define LED_PIN             4   // 2560 PIN 1
+
+//////////////////////////
+// LCDs and Controllers //
+//////////////////////////
+
+//
+// LCD Display output pins
+//
+
+// Requires #define U8GLIB_SH1106_EINSTART in Configuration.h
+// u8glib constructor
+// U8GLIB_SH1106_128X64 u8g(DOGLCD_SCK, DOGLCD_MOSI, DOGLCD_CS, LCD_PINS_DC, LCD_PINS_RS);
+
+#define LCD_PINS_DC        78   // 2560 PIN 4
+#define LCD_PINS_RS        79   // 2560 PIN 8
+// DOGM SPI LCD Support
+#define DOGLCD_CS           3   // 2560 PIN 7
+#define DOGLCD_MOSI         2   // 2560 PIN 6
+#define DOGLCD_SCK          5   // 2560 PIN 5
+#define DOGLCD_A0           2   // 2560 PIN 6
+
+//
+// LCD Display input pins
+//
+#define BTN_UP             25   // 2560 PIN 75
+#define BTN_DWN            26   // 2560 PIN 74
+#define BTN_LFT            27   // 2560 PIN 73
+#define BTN_RT             28   // 2560 PIN 72
+
+// 'OK' button
+#define BTN_ENC            29   // 2560 PIN 71
+
+// Set Kill to right arrow, same as RIGID_PANEL
+#define KILL_PIN           28   // 2560 PIN 72