Add initial FuseSoC support

[olofk]

This adds support for running many (but not all) testbenches using
FuseSoC. This also makes it possible for user projects to depend
on the core

Quick instructions

#Install FuseSoC
pip3 install fusesoc

#Create and enter a new workspace
mkdir workspace && cd workspace

#Add PDK as a library in the workspace
fusesoc library add sky130_pdk https://github.com/olofk/pdklite

#If caravel repo is available locally, register as a library in the workspace with
fusesoc library add caravel /path/to/caravel
#...or...
fusesoc library add caravel https://github.com/efabless/caravel
# ..to get the upstream repo from github

#To run the hkspi testbench
fusesoc run --target=hkspi_tb efabless:openmpw:caravel 

#Run with modelsim instead of default tool (icarus). Likely works with e.g. xsim, vcs, xcelium and rivierapro as well once #57 is fixed
fusesoc run --target=hkspi_tb --tool=modelsim efabless:openmpw:caravel

#List all targets
fusesoc core show efabless:openmpw:caravel