Add a flush method to the hardware depended functions. It is called whenever

a sequence of characters or commands have to be terminated (common_init,
position, flush).  Modify the lcd2usb subdriver to implement a small command
buffer and implement flush().

server/drivers/hd44780-lcd2usb.h

@@ -8,6 +8,9 @@
 #define LCD2USB_VENDORID	0x0403
 #define LCD2USB_PRODUCTID	0xc630
 
+/* current protocol supports up to 4 bytes */
+#define LCD2USB_MAX_CMD		4
+
 /* target is a bit map for CMD/DATA */
 #define LCD2USB_CTRL_0		(1<<3)
 #define LCD2USB_CTRL_1		(1<<4)