some useful options for vcd converter, some docs

Makefile

@@ -38,7 +38,11 @@ jpeg_rtl_sim: $(JPEG_CORE_VVP)
 # picorv32 targets
 #
 
-picorv32_rtl_sim:
+test_vcd:
 	make -C picorv32 test_vcd
-	cp picorv32/testbench.vcd picorv32.vcd
 
+test_ez_vcd:
+	make -C picorv32 test_ez_vcd
+
+testbench.npy:
+	./picorv32_vcd_import.py picorv32/testbench.vcd testbench.npy