Add new variant for Digilent Nexys4 (Artix 7)

Remember to set Propeller tool to use RTS as the reset signal when using
the Nexys4 board. (There's no DTR in the built-in UART.)
Preparing for merge of Nexys4

Andrewsil1 created a new Xilinx target Nexys4, but it was based on an
earlier version of Magnus Karlsson's branch, which has since been
reverted. I merged Magnus' latest changes into Andy's tree, which also
replaced tabs by spaces.