verilator: handle VERILATOR_ROOT correctly

If you want to run Verilator from the kit directory,
VERILATOR_ROOT need to point to the Verilator source directory.

If you have installed Verilator globaly, using the normal
system paths (with unset VERILATOR_ROOT && ./configure)
you don't have anything to do (just to make sure Verilator
is in your PATH).

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>