Add a --start_seed argument to core_ibex/sim.py
Add a --start_seed argument to core_ibex/sim.py

If --iterations is 1, this is equivalent to the existing --seed
argument (which we're keeping unchanged). If --iterations is
0 (reading iteration counts from the config) or positive, successive
test iterations use successive seeds. So if you pass --start_seed 123
and run ten iterations, they will run with seeds 123, 124, ... through
133.

Lots of the added code is to check that you don't do something silly
like --seed=123 --iterations=10. Since the next patch will convert the
Makefile which runs this script to using --start_seed, that's all dead
code. Maybe we should get rid of that argument at some point.

AmbiML/ibex,lowRISC/ibex,lowRISC/ibex,AmbiML/ibex,lowRISC/ibex,lowRISC/ibex,AmbiML/ibex,AmbiML/ibex