add arty s7 board and a counter example for it

[MaksRawski]

I've basically added arty s7 board by extending what @liibin has done for ax7050 and the counter example worked just fine. After copying the newly generated arty s7 pinmap, which was in path/to/f4pga-arch-defs/build/xilinx/xc7/arty-s7_pinmap.csv, to $F4PGA_DIR/xc7/share/f4pga/arch/xc7s50_test/xc7s50csga324-1/pinmap.csv I was even able to build the counter example outside this repo using the common.mk from f4pga-examples.