Use 'false'

Marlin/src/HAL/LPC1768/HAL.cpp

 }
 
 void flashFirmware(const int16_t) {
-  USB_Connect(FALSE);  // USB clear connection
+  USB_Connect(false);  // USB clear connection
   delay(2000);         // Give OS time to notice
   NVIC_SystemReset();
 }

Marlin/src/HAL/LPC1768/main.cpp

   #endif
 
   USB_Init();                               // USB Initialization
-  USB_Connect(FALSE);                       // USB clear connection
+  USB_Connect(false);                       // USB clear connection
   delay(1000);                              // Give OS time to notice
-  USB_Connect(TRUE);
+  USB_Connect(true);
 
   #if HAS_SD_HOST_DRIVE
     MSC_SD_Init(0);                         // Enable USB SD card access