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>
verilator: wip
verilator: add source files to LDFLAGS

this is to append the libs to the end of link command.
Franck already have this included in a pull request, so this is
just a temporary solution ontop of the other of his changes I
have in this branch.