xilinx_bscan_spi: port to new migen and clean-up

xilinx_bscan_spi: port to new migen and clean-up

* port to new migen
* streamline package/part specification
* add pullup (Series3, Series6) and pullnone (Series7) for unused pins
  as xilinx impact/vivado do it.
* specify respective toolchains
* build Series7 with vivado (broader support, faster)
* point to prebuilt bitstreams at https://github.com/jordens/bscan_spi_bitstreams

Change-Id: Ibfef3d78f855b754425f3e6131e2e49fa111e09a
Signed-off-by: Robert Jordens <jordens@gmail.com>
Reviewed-on: http://openocd.zylin.com/3173
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins
Reviewed-by: Robert Jördens
Reviewed-by: William D. Jones
Reviewed-by: Tim "mithro" Ansell <mithro@mithis.com>