utils: add helpers for finding verilator and VERILATOR_ROOT

find_verilator code is factored out from verilator.py.
get_verilator_root relies on the fact that verilator will print
the hardcoded value for VERILATOR_ROOT when fed '-V' as an
argument.
verilator: use find_verilator() helper
sections: use get_verilator_root to obtain VERILATOR_ROOT