Start clean-up of JTAG driver interface:

- Factor jtag_add_scan_check to call new jtag_add_scan_check_alloc helper.
- Use conditional logic to define two versions of the helper.
- These helpers will be moved to other files in future patches.


git-svn-id: svn://svn.berlios.de/openocd/trunk@1989 b42882b7-edfa-0310-969c-e2dbd0fdcd60