Fix and optimize MightyBoard (#20493)

Marlin/Configuration_adv.h

 /**
  * I2C-based DIGIPOTs (e.g., Azteeg X3 Pro)
  */
-//#define DIGIPOT_MCP4018             // Requires https://github.com/stawel/SlowSoftI2CMaster
+//#define DIGIPOT_MCP4018             // Requires https://github.com/felias-fogg/SlowSoftI2CMaster
 //#define DIGIPOT_MCP4451
 #if EITHER(DIGIPOT_MCP4018, DIGIPOT_MCP4451)
   #define DIGIPOT_I2C_NUM_CHANNELS 8  // 5DPRINT:4   AZTEEG_X3_PRO:8   MKS_SBASE:5   MIGHTYBOARD_REVE:5

Marlin/src/feature/digipot/digipot_mcp4018.cpp

 #include "digipot.h"
 
 #include <Stream.h>
-#include <SlowSoftI2CMaster.h>  // https://github.com/stawel/SlowSoftI2CMaster
+#include <SlowSoftI2CMaster.h>  // https://github.com/felias-fogg/SlowSoftI2CMaster
 
 // Settings for the I2C based DIGIPOT (MCP4018) based on WT150
 
 }
 
 static SlowSoftI2CMaster pots[DIGIPOT_I2C_NUM_CHANNELS] = {
-  SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_X, DIGIPOTS_I2C_SCL)
+  SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_X, DIGIPOTS_I2C_SCL, ENABLED(DIGIPOT_ENABLE_I2C_PULLUPS))
   #if DIGIPOT_I2C_NUM_CHANNELS > 1
-    , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_Y, DIGIPOTS_I2C_SCL)
+    , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_Y, DIGIPOTS_I2C_SCL, ENABLED(DIGIPOT_ENABLE_I2C_PULLUPS))
     #if DIGIPOT_I2C_NUM_CHANNELS > 2
-      , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_Z, DIGIPOTS_I2C_SCL)
+      , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_Z, DIGIPOTS_I2C_SCL, ENABLED(DIGIPOT_ENABLE_I2C_PULLUPS))
       #if DIGIPOT_I2C_NUM_CHANNELS > 3
-        , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_E0, DIGIPOTS_I2C_SCL)
+        , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_E0, DIGIPOTS_I2C_SCL, ENABLED(DIGIPOT_ENABLE_I2C_PULLUPS))
         #if DIGIPOT_I2C_NUM_CHANNELS > 4
-          , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_E1, DIGIPOTS_I2C_SCL)
+          , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_E1, DIGIPOTS_I2C_SCL, ENABLED(DIGIPOT_ENABLE_I2C_PULLUPS))
           #if DIGIPOT_I2C_NUM_CHANNELS > 5
-            , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_E2, DIGIPOTS_I2C_SCL)
+            , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_E2, DIGIPOTS_I2C_SCL, ENABLED(DIGIPOT_ENABLE_I2C_PULLUPS))
             #if DIGIPOT_I2C_NUM_CHANNELS > 6
-              , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_E3, DIGIPOTS_I2C_SCL)
+              , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_E3, DIGIPOTS_I2C_SCL, ENABLED(DIGIPOT_ENABLE_I2C_PULLUPS))
               #if DIGIPOT_I2C_NUM_CHANNELS > 7
-                , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_E4, DIGIPOTS_I2C_SCL)
+                , SlowSoftI2CMaster(DIGIPOTS_I2C_SDA_E4, DIGIPOTS_I2C_SCL, ENABLED(DIGIPOT_ENABLE_I2C_PULLUPS))
               #endif
             #endif
           #endif

Marlin/src/pins/mega/pins_MIGHTYBOARD_REVE.h

 #ifndef DIGIPOT_I2C_ADDRESS_A
   #define DIGIPOT_I2C_ADDRESS_A             0x2F  // unshifted slave address (5E <- 2F << 1)
 #endif
+#define DIGIPOT_ENABLE_I2C_PULLUPS                // MightyBoard doesn't have hardware I2C pin pull-ups.
 
 //
 // Temperature Sensors

platformio.ini

 [env:mega2560ext]
 platform            = atmelavr
 extends             = env:mega2560
-board               = megaatmega2560
 board_build.variant = megaextendedpins
 extra_scripts       = ${common.extra_scripts}
                       pre:buildroot/share/PlatformIO/scripts/copy_marlin_variant_to_framework.py
 board    = megaatmega1280
 
 #
-# MightyBoard ATmega2560 (MegaCore 100 pin boards variants)
+# MightyBoard AVR with extended pins
+#
+[mega_extended_optimized]
+extends             = common_avr8
+board_build.variant = megaextendedpins
+extra_scripts       = ${common.extra_scripts}
+                      pre:buildroot/share/PlatformIO/scripts/copy_marlin_variant_to_framework.py
+upload_speed        = 57600
+build_flags         = ${common.build_flags} -fno-tree-scev-cprop -fno-split-wide-types -Wl,--relax -mcall-prologues
+
+#
+# MightyBoard ATmega1280
 #
 [env:MightyBoard1280]
-platform      = atmelavr
-extends       = common_avr8
-board         = ATmega1280
-upload_speed  = 57600
+platform = atmelavr
+extends  = mega_extended_optimized
+board    = megamega1280
 
 #
-# MightyBoard ATmega2560 (MegaCore 100 pin boards variants)
+# MightyBoard ATmega2560
 #
 [env:MightyBoard2560]
-platform                  = atmelavr
-extends                   = common_avr8
-board                     = ATmega2560
-upload_protocol           = wiring
-upload_speed              = 57600
-board_upload.maximum_size = 253952
+platform = atmelavr
+extends  = mega_extended_optimized
+board    = megaatmega2560
 
 #
 # RAMBo