Added submodule with Yosys and integrated it with the LiteX minitest

Signed-off-by: Maciej Kurc <mkurc@antmicro.com>

.gitmodules

@@ -19,3 +19,6 @@
 [submodule "third_party/python-sdf-timing"]
 	path = third_party/python-sdf-timing
 	url = https://github.com/SymbiFlow/python-sdf-timing.git
+[submodule "third_party/yosys"]
+	path = third_party/yosys
+	url = https://github.com/YosysHQ/yosys

minitests/litex/src.yosys/Makefile

@@ -1,3 +1,5 @@
+YOSYS_PATH = $(abspath ../../../third_party/yosys)
+YOSYS = $(YOSYS_PATH)/yosys
 VIVADO = /opt/Xilinx/Vivado/2017.2/bin/vivado
 
 all: top.bit
@@ -10,6 +12,9 @@ clean:
 
 .PHONY: all clean
 
+$(YOSYS):
+	cd $(YOSYS_PATH) && make config-gcc && make -j$(shell nproc)
+
 top.edif: $(YOSYS)
 	$(YOSYS) -s synth.ys -l yosys.log
 

third_party/yosys

@@ -0,0 +1 @@
+Subproject commit d4f77d408c754285969969bda3a6985c1fbe9fb6