Cocotb: Recommend checking for TOPLEVEL

[raysalemi]

If TOPLEVEL is not set iverilog fails with an obscure error message because there is nothing after the -s option. I recommend that Makefile.sim check for TOPLEVEL and (and other required variables) and give a clean error message.

[ktbarrett]

This issue is specific to Icarus. Other simulators allow TOPLEVEL to be empty, which in cocotb will return the root handle as the dut object, and not some entity.

[haroonshafique-10xe]

This issue is specific to Icarus. Other simulators allow TOPLEVEL to be empty, which in cocotb will return the root handle as the dut object, and not some entity.

@ktbarrett I tried running it without setting TOPLEVEL for verilator as well, and it failed too because of missing toplevel module name for option --top-module in verilator command.

[ktbarrett]

Thanks to @haroonshafique-10xe this was closed by #2547.