OMAP3 zoom2 Use usbtty if the debug board is not connected.

The preferred serial output comes from the debug board.
When the debug board is disconnected, fall back on using
usbtty from the usb connector on the Zoom2 board.

This shows up as /dev/ttyACM0 in a linux host.

Signed-off-by: Tom Rix <Tom.Rix@windriver.com>
diff --git a/board/logicpd/zoom2/zoom2_serial.c b/board/logicpd/zoom2/zoom2_serial.c
index a3d777d..ba58e39 100644
--- a/board/logicpd/zoom2/zoom2_serial.c
+++ b/board/logicpd/zoom2/zoom2_serial.c
@@ -86,6 +86,8 @@
 			quad_putc_dev (base, '\r');
 
 		NS16550_putc ((NS16550_t) base, c);
+	} else {
+		usbtty_putc(c);
 	}
 }
 
@@ -94,6 +96,8 @@
 	if (zoom2_debug_board_connected ()) {
 		while ((s != NULL) && (*s != '\0'))
 			quad_putc_dev (base, *s++);
+	} else {
+		usbtty_puts(s);
 	}
 }
 
@@ -101,16 +105,16 @@
 {
 	if (zoom2_debug_board_connected ())
 		return NS16550_getc ((NS16550_t) base);
-	else
-		return 0;
+
+	return usbtty_getc();
 }
 
 int quad_tstc_dev (unsigned long base)
 {
 	if (zoom2_debug_board_connected ())
 		return NS16550_tstc ((NS16550_t) base);
-	else
-		return 0;
+
+	return usbtty_tstc();
 }
 
 void quad_setbrg_dev (unsigned long base)