JTAG: simple autoprobing

This patch adds basic autoprobing support for the JTAG scan chains
which cooperate.  To use, you can invoke OpenOCD with just:

 - interface spec: "-f interface/...cfg"
 - possibly with "-c 'reset_config ...'" for SRST/TRST
 - possibly with "-c 'jtag_khz ...'" for the JTAG clock

Then set up config files matching the reported TAPs.  It doesn't
declare targets ... just TAPs.  So facilities above the JTAG and
SVF/XSVF levels won't be available without a real config; this is
almost purely a way to generate diagnostics.

Autoprobe was successful with most boards I tested, except ones
incorporating C55x DSPs (which don't cooperate with this scheme
for IR length autodetection).  Here's what one multi-TAP chip
reported, with the "Warn:" prefixes removed:

 clock speed 500 kHz
 There are no enabled taps.  AUTO PROBING MIGHT NOT WORK!!
 AUTO auto0.tap - use "jtag newtap auto0 tap -expected-id 0x2b900f0f ..."
 AUTO auto1.tap - use "jtag newtap auto1 tap -expected-id 0x07926001 ..."
 AUTO auto2.tap - use "jtag newtap auto2 tap -expected-id 0x0b73b02f ..."
 AUTO auto0.tap - use "... -irlen 4"
 AUTO auto1.tap - use "... -irlen 4"
 AUTO auto2.tap - use "... -irlen 6"
 no gdb ports allocated as no target has been specified

The patch tweaks IR setup a bit, so we can represent TAPs with
undeclared IR length.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

TODO

@@ -55,8 +55,10 @@ directly in minidriver API for better embedded host performance.
 
 The following tasks have been suggested for adding new core JTAG support:
 
-- autodetect devices present on the scan chain
-  - implement 'discover_taps' command
+- Improve autodetection of TAPs by supporting tcl escape procedures that
+  can configure discovered TAPs based on IDCODE value ... they could:
+    - Remove guessing for irlen
+    - Allow non-default irmask/ircapture values
 - SPI/UART emulation:
   - (ab)use bit-banging JTAG interfaces to emulate SPI/UART
   - allow SPI to program flash, MCUs, etc.
@@ -94,6 +96,8 @@ interface support:
 - FT2232 (libftdi):
   - make performance comparable to alternatives (on Win32, D2XX is faster)
   - make usability comparable to alternatives
+- Autodetect USB based adapters; this should be easy on Linux.  If there's
+  more than one, list the options; otherwise, just select that one.
 
 The following tasks have been suggested for adding new JTAG interfaces:
 
@@ -133,6 +137,7 @@ Once the above are completed:
 
 @section thelisttargets Target Support
 
+- Many common ARM cores could be autodetected using IDCODE
 - general layer cleanup: @par
   https://lists.berlios.de/pipermail/openocd-development/2009-May/006590.html
 - regression: "reset halt" between 729(works) and 788(fails): @par