Example for Kintex7 on Digilent Genesys2 board (stub)

[unbtorsten]

This is a first shot at adding Kintex7 to the examples Digilent's Genesys2 board (xc7k325tffg900-2 package).
Writing up the steps should be helpful in finding consensus for how the actual examples need to be set up.

Note that at this time build process fails:

$ TARGET="genesys2" make -C counter_test
make: Entering directory 'f4pga-examples/xc7/counter_test'
cd f4pga-examples/xc7/counter_test/build/genesys2 && symbiflow_synth -t top -v f4pga-examples/xc7/counter_test/counter_genesys2.v -d kintex7 -p xc7k325tffg900-2 -x f4pga-examples/xc7/counter_test/genesys2.xdc 2>&1 > /dev/null
cd f4pga-examples/xc7/counter_test/build/genesys2 && symbiflow_pack -e top.eblif -d xc7k325t_test  2>&1 > /dev/null
Error 1: opt/f4pga/xc7/install/share/symbiflow/arch/xc7k325t_test/arch.timing.xml:-1 Failed to open file
make: *** [f4pga-examples/xc7/counter_test/../../common/common.mk:63: f4pga-examples/xc7/counter_test/build/genesys2/top.net] Error 1
make: Leaving directory 'f4pga-examples/xc7/counter_test'

This PR can serve as reference to add the corresponding architecture in https://github.com/SymbiFlow/f4pga-arch-defs