Add private src/jtag/interface.h for use by JTAG interface drivers:

- Move the jtag_interface structure definition.
- Move the Cable API declarations.
- Add new header file to automake input.

The next patch will move the implementation to interface.c.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2008 b42882b7-edfa-0310-969c-e2dbd0fdcd60