utils: make get_verilator_root work with python3
utils: make get_verilator_root work with python3

str and byte string handling in python2.7 and python3.x differ
a lot, doing str type of operations on a byte string in python3
doesn't fly.
To fix this, decode the byte string into a str before doing
operations on it.

olofk/fusesoc,olofk/fusesoc,lowRISC/fusesoc,lowRISC/fusesoc