ARM: get usb_serial.c in.

Once more just made sure it doesn't accidentially slip in without
giving the user a warning.

Makefile-ARM

@@ -101,9 +101,8 @@ SOURCES  = mendel.c cpu.c serial.c sermsg.c sersendf.c delay.c
 SOURCES += gcode_parse.c gcode_process.c pinio.c timer.c clock.c
 SOURCES += dda_queue.c dda_maths.c dda_kinematics.c dda.c dda_lookahead.c
 SOURCES += analog.c temp.c heater.c home.c debug.c crc.c pff.c pff_diskio.c
-SOURCES += spi.c intercom.c graycode.c
+SOURCES += spi.c intercom.c graycode.c usb_serial.c
 # Sources left:
-# usb_serial.c
 # watchdog.c
 
 ifeq ($(MCU), lpc1114)

usb_serial.h

@@ -3,6 +3,13 @@
 
 #include <stdint.h>
 
+/**
+  Test configuration.
+*/
+#ifdef __ARMEL__
+  #error Direct USB connection (USB_SERIAL) not yet supported on ARM.
+#endif
+
 // setup
 void usb_init(void);			// initialize everything
 uint8_t usb_configured(void);		// is the USB port configured