#163 Use Xilinx .bit file with "jtagspi" flash driver

[cioma]

In OpenOCD v0.10 it's possible to use "jtagspi" flash driver to program SPI configuration flash attached to a Xilinx FPGA. It requires .bin configuration file to be provided (i.e. .bit file without header). It would be great to be able to use .bit file directly.