This adds Verilator lint support to our fusesoc core file. A waiver file
is created to waive all well-understood lint warnings. The UNOPTFLAT
warnings are not well understood at the moment, they are waived for now
and further discussion is expected to happen in a GH issue (referenced
in the waiver).
Run with
```
fusesoc --cores-root . run --target=lint lowrisc:ibex:ibex_core
```
The waiver file support requires edalize >= 0.1.5.
