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/SoftwareSerial.cpp

@@ -29,6 +29,7 @@ The latest version of this library can always be found at
 http://arduiniana.org.
 */
 
+#ifdef TARGET_LPC1768
 
 //
 // Includes
@@ -396,3 +397,5 @@ int SoftwareSerial::peek()
   // Read from "head"
   return _receive_buffer[_receive_buffer_head];
 }
+
+#endif // TARGET_LPC1768