arty: Add an option to enable jtagbone

Then adds jtagbone for arty.  I have tested with the following
litex_server and it seems to work fine.

  litex_server --jtag --jtag-config openocd_xc7_ft2232.cfg

Note, the jtagbone and etherbone may be mutually exclusive, but I am not
sure how to define that in the args.