fpga: tool to download bitfile to xilinx 7-series fpga

- Currently only works if the FPGA is unprogrammed, out of reset.
- Does not have the entire 7-series idcodes in its table, so will
  not recognize all parts.
- Does not do any bitfile metadata vs device validation yet, so will
  happily send the wrong bitfile to a device (but that should always
  fail).