* generic: Fix Python path not including the examples/ directory in the example
It seems that at some point, the embedded Python stopped including the
directory that nextpnr-generic was executed in inside of its sys.path.
This likely happened by de3d5be8 specifying an explicit argv to
the interpreter init function.
* generic: Improve the example for K != 4
Configuring K < 4 used to result in "dangling" inputs to the cells being
generated (those are just not driven by anything in the resulting Verilog/JSON).
Configuring K > 4 used to result in an assertion crash in cells.cc.
The ctx.setLutK call fixes both cases.
