Improve use of automake conditionals for FTDI-based JTAG drivers:

- Remove once-used XXX_FTD2XX symbols; replace with XXX_DRIVER symbols.
- Enabled when either libftdi or FTD2xx driver should be built.
- Eliminates redundant DRIVERSFILE assignment in JTAG automake input.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2177 b42882b7-edfa-0310-969c-e2dbd0fdcd60