Adding support for a new cable to urjtag.

OK, you've got a JTAG cable (computer-to-jtag interface) that isn't
yet supported by urjtag.  This is a guide for would-be urjtag
contributors on how to add support for new cables.

There are three basic classes of cables, based on how they connect to
the host computer:  Parallel port, USB, and Other.


Among cables with a USB interface, there is a loose distinction
between cables based on FTDI chips and all of the rest.


Adding a new USB/FTDI cable.

If you are sure that your cable is identical to an existing supported
cable based on an FTDI chip except for having different USB Vendor and
Product ID codes, you might not have to modify urjtag at all.  You can
override the VID and PID like this:

   jtag> cable ARM-OCD-USB vid=0xabcd pid=0x1234

You can make this cable more convenient by teaching urjtag to
recognize that cable's vendor and product ID.  Do this by adding a
line to src/tap/cable/ft2232.c containing a URJ_DECLARE_FTDX_CABLE()
macro call.  Follow the examples near the bottom of ft2232.c.



If you FTDIchip-based cable isn't wired the same as a an existing
cable, you'll need to add a whole new cable driver.

To add a whole new ft2232 cable driver, you'll need to make
several modifications.  The rest of this example is based on
the new cable "FT4232", a generic cable using FTDI's FT4232 chip.

1. add a new *_init() and possibly a new ft2232_*_done()
function definition to src/tap/cable/ft2232.c
For the 4232, we'll add ft4232_generic_init().

2. add a new "const urj_cable_driver_t urj_tap_cable_ft2232_*_driver =
{...};" declaration near the bottom of src/tap/cable/ft2232.c As you
can see from the existing declarations, each one must have a different
name.  The contents of this structure is one place that the name of
the cable appears.  Most of these entries differ in the "init" and and
sometimes the "done" entries.

For the 4232, we add "urj_tap_cable_ft2232_ft4232_driver"

3. Add a _URJ_CABLE entry to tap/cable_list.h
The argument to the macro is the unique part of the urj_cable_driver_t
name added to ft2232.c in step 2 above. For example
     _URJ_CABLE(ft2232_ft4232)
goes with
     extern const urj_cable_driver_t urj_tap_cable_ft2232_ft4232_driver

4. Optionally add a URJ_DECLARE_FTDX_CABLE() macro near the bottom of
ft2232.c, to provide a default VID/PID to look for on the bus to find
the cable.  This macro defines an entry included in a list of
ftdichip USB drivers that is used to look up defaults.

5. If you did step 4, finish it by adding an entry to
src/tap/cable/generic_usbconn_list.h.   Each entry is a _URJ_USB_FTDX()
macro.  The one argument to the _URJ_USB_FTDX() macro is the last argument to
the URJ_DECLARE_FTDX_CABLE.