Added BT support on AT90USB devices

Added AT90USB device BT connectivity support using second UART.

Marlin/Configuration.h

 // This allows the connection of wireless adapters (for instance) to non-default port pins.
 // Serial port 0 is still used by the Arduino bootloader regardless of this setting.
 #define SERIAL_PORT 0
-//#define SERIAL_PORT 1  // Define serial port 1 for bluetooth configuration in AT90USB configurations
 
 // This determines the communication speed of the printer
 // This determines the communication speed of the printer
 #define BAUDRATE 250000
 
 // This enables the serial port associated to the Bluetooth interface
-//#define BTENABLED              // Enable BT interface
-
-
-// SERIAL_PORT selects which serial port should be used for communication with the host.
-// This allows the connection of wireless adapters (for instance) to non-default port pins.
-// Serial port 0 is still used by the Arduino bootloader regardless of this setting.
-#define SERIAL_PORT 0
+//#define BTENABLED              // Enable BT interface on AT90USB devices
 
 
 //// The following define selects which electronics board you have. Please choose the one that matches your setup
 // 21 = Elefu Ra Board (v3)
 
 #ifndef MOTHERBOARD
-//[FMC]#define MOTHERBOARD 7
-#define MOTHERBOARD 83
+//#define MOTHERBOARD 7
 #endif
 
 // Define this to set a custom name for your generic Mendel,
 
 // Shift register panels
 // ---------------------
+// 2 wire Non-latching LCD SR from:
+// https://bitbucket.org/fmalpartida/new-liquidcrystal/wiki/schematics#!shiftregister-connection 
 //#define SR_LCD
 #ifdef SR_LCD
    #define SR_LCD_2W_NL    // Non latching 2 wire shiftregister

Marlin/Marlin.h

 
 #ifdef AT90USB
    #ifdef BTENABLED
-         extern HardwareSerial bt;
          #define MYSERIAL bt
    #else
          #define MYSERIAL Serial
    #endif // BTENABLED
 #endif
 
-#ifdef AT90USB
-  #define MYSERIAL Serial
-#else
-  #define MYSERIAL MSerial
-#endif
-
 #define SERIAL_PROTOCOL(x) (MYSERIAL.print(x))
 #define SERIAL_PROTOCOL_F(x,y) (MYSERIAL.print(x,y))
 #define SERIAL_PROTOCOLPGM(x) (serialprintPGM(PSTR(x)))