Fusesoc

[olofk]

First commits just adds a FuseSoC core description. Second commit also adds a generator that can be used by cores that depend on axi_node. Some example usage:

1. Add axi_node as a fusesoc library fusesoc library add axi_node <path> where is the dir of a local checkout or a remote git repo
2. Check it's there. fusesoc core show pulp-platform.org::axi_node should show some details of the location and available targets
3. Run verilator in lint-mode on the core fusesoc run --target=lint pulp-platform.org::axi_node
4. Get some info on the generator fusesoc gen show axi_intercon_gen

Tested on a SoC design with Verilator sin and Vivado synthesis

[zarubaf]

ping @accuminium