Make use of the new usb_serial (and drop usage of lufa_serial).

At this point in time not a single user of LUFA is known and
here is reported how usb_serial apparently works more reliable:

http://forums.reprap.org/read.php?147,33082,160724#msg-160724

So it's likely we can move from lufa_serial to usb_serial entirely.

Makefile

@@ -94,7 +94,7 @@ PROGID = stk500v2
 
 PROGRAM = mendel
 
-SOURCES = $(PROGRAM).c gcode_parse.c gcode_process.c dda.c dda_maths.c dda_queue.c timer.c temp.c sermsg.c watchdog.c debug.c sersendf.c heater.c analog.c intercom.c pinio.c clock.c home.c crc.c delay.c
+SOURCES = $(PROGRAM).c gcode_parse.c gcode_process.c dda.c dda_maths.c dda_queue.c timer.c temp.c sermsg.c watchdog.c debug.c sersendf.c heater.c analog.c intercom.c pinio.c clock.c home.c crc.c delay.c serial.c
 
 ARCH = avr-
 CC = $(ARCH)gcc
@@ -110,19 +110,14 @@ LIBDEPS =
 SUBDIRS =
 
 ifneq (,$(findstring usb,$(MCU_TARGET)))
-USE_LUFA = true
+USE_USB = true
 endif
 ifneq (,$(findstring u4,$(MCU_TARGET)))
-USE_LUFA = true
+USE_USB = true
 endif
-ifdef USE_LUFA
-LDFLAGS += -Llufa_serial
-
-LIBS += -llufa_serial
-SUBDIRS += lufa_serial
-LIBDEPS += lufa_serial/liblufa_serial.a
-else
-SOURCES += serial.c
+ifdef USE_USB
+CFLAGS += -DUSE_USB
+SOURCES += usb_serial.c
 endif
 
 ifeq ($(PROGBAUD),0)

serial.c

@@ -24,6 +24,7 @@
 /// ascii XON character
 #define		ASCII_XON		17
 
+#ifndef USE_USB
 /// rx buffer head pointer. Points to next available space.
 volatile uint8_t rxhead = 0;
 /// rx buffer tail pointer. Points to last character in buffer
@@ -223,6 +224,7 @@ void serial_writechar(uint8_t data)
 	// enable TX interrupt so we can send this character
 	UCSR0B |= MASK(UDRIE0);
 }
+#endif /* USE_USB */
 
 /// send a whole block
 void serial_writeblock(void *data, int datalen)

serial.h

@@ -5,10 +5,17 @@
 #include	<avr/io.h>
 #include	<avr/pgmspace.h>
 
+#ifdef USE_USB
+  #define serial_init() usb_init()
+  #define serial_rxchars() usb_serial_available()
+  #define serial_popchar() usb_serial_getchar()
+  #define serial_writechar(c) usb_serial_putchar(c)
+#else
   // initialise serial subsystem
   void serial_init(void);
 
-// return number of characters in the receive buffer, and number of spaces in the send buffer
+  // return number of characters in the receive buffer,
+  // and number of spaces in the send buffer
   uint8_t serial_rxchars(void);
   // uint8_t serial_txchars(void);
 
@@ -16,6 +23,7 @@ uint8_t serial_rxchars(void);
   uint8_t serial_popchar(void);
   // send one character
   void serial_writechar(uint8_t data);
+#endif
 
 // read/write many characters
 // uint8_t serial_recvblock(uint8_t *block, int blocksize);