Add needed platform defined gaurds to compile with Arduino IDE

Also removed explicit platform HAL directory inclusion from platformio.ini to make sure these errors are caught by Travis in the future

Marlin/src/HAL/HAL_LPC1768/HardwareSerial.cpp

@@ -20,6 +20,8 @@
  *
  */
 
+#ifdef TARGET_LPC1768
+
 #include "../../../macros.h"
 #include "../HAL.h"
 #include "HardwareSerial.h"
@@ -634,3 +636,5 @@ void UART3_IRQHandler (void)
 #ifdef __cplusplus
 }
 #endif
+
+#endif // TARGET_LPC1768