Move the JTAG cable interface API implementation

- Cloned the src/jtag/jtag.c file to src/jtag/interface.c.
- For each for of those files, deleted the contents of the other.
- Add new source file to automake input.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2012 b42882b7-edfa-0310-969c-e2dbd0fdcd60