add jtag/usb_common.[ch] files

Begins to consolidate code used by several USB JTAG interfaces.
This first patch provides the required build system changes and
a common jtag_usb_open routine, which will replace the guts for
probing the busses and devices for possible VID/PID matches.
The following patches convert each driver to use it.

configure.in

@@ -996,6 +996,7 @@ if test $build_jlink = yes -o $build_vsllink = yes -o $build_usbprog = yes -o \
 then
   AC_CHECK_HEADERS([usb.h],[],
   [AC_MSG_ERROR([usb.h is required to build some OpenOCD driver(s)])])
+  build_usb=yes
 fi
 
 AM_CONDITIONAL(RELEASE, test $build_release = yes)
@@ -1021,6 +1022,7 @@ AM_CONDITIONAL(JLINK, test $build_jlink = yes)
 AM_CONDITIONAL(VSLLINK, test $build_vsllink = yes)
 AM_CONDITIONAL(RLINK, test $build_rlink = yes)
 AM_CONDITIONAL(ARMJTAGEW, test $build_armjtagew = yes)
+AM_CONDITIONAL(USB, test $build_usb = yes)
 AM_CONDITIONAL(IS_CYGWIN, test $is_cygwin = yes)
 AM_CONDITIONAL(IS_MINGW, test $is_mingw = yes)
 AM_CONDITIONAL(IS_WIN32, test $is_win32 = yes)