Rename BOARD_STM32F4 to fix use with MB()

Marlin/src/core/boards.h

 //
 
 #define BOARD_BEAST                   4200  // STM32F4xxVxT6 Libmaple-based STM32F4 controller
-#define BOARD_STM32F4                 4201  // STM32 STM32GENERIC-based STM32F4 controller
+#define BOARD_GENERIC_STM32F4         4201  // STM32 STM32GENERIC-based STM32F4 controller
 #define BOARD_ARMED                   4202  // Arm'ed STM32F4-based controller
 #define BOARD_RUMBA32                 4203  // RUMBA32 STM32F4-based controller
 #define BOARD_BLACK_STM32F407VE       4204  // BLACK_STM32F407VE

Marlin/src/pins/pins.h

 
 #elif MB(BEAST)
   #include "stm32/pins_BEAST.h"                 // STM32F4                                env:STM32F4
-#elif MB(STM32F4)
-  #include "stm32/pins_STM32F4.h"               // STM32F4                                env:STM32F4
+#elif MB(GENERIC_STM32F4)
+  #include "stm32/pins_GENERIC_STM32F4.h"       // STM32F4                                env:STM32F4
 #elif MB(ARMED)
   #include "stm32/pins_ARMED.h"                 // STM32F4                                env:ARMED
 #elif MB(RUMBA32)

config/examples/STM32/STM32F4/Configuration.h

 
 // Choose the name from boards.h that matches your setup
 #ifndef MOTHERBOARD
-  #define MOTHERBOARD BOARD_STM32F4
+  #define MOTHERBOARD BOARD_GENERIC_STM32F4
 #endif
 
 // Name displayed in the LCD "Ready" message and Info menu