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.

This is a replacement to https://github.com/olofk/fusesoc/pull/39
and contains the fix I propose there, and as Franck suggests there,
I'm preparing the patch for it with a proper commit message here.