Add yosys/verilator support

Add yosys/verilator support

Add a microwatt-verilator target that simulates the
ghdl -> yosys -> verilog -> verilator path. A good test of
ghdl/yosys synthesis.

Because the everything is run through synthesis, the instruction
image is baked into the build via the RAM_INIT_FILE generic.

Signed-off-by: Anton Blanchard <anton@linux.ibm.com>