Add SYNTH_ECP5_FLAGS option for building

This is useful to specify "-noflatten" which helps CI stay under 8GB
limit.

Normally the AUTONAME stage of yosys will take around 10GB if
operating on the whole design. With -noflatten, AUTONAME occurs only
per VHDL entity, so only consumes around 3GB of memory. This gets us
under the limitations on github actions.

More discussion here:
  https://github.com/antonblanchard/microwatt/pull/209#issuecomment-652186078

Signed-off-by: Michael Neuling <mikey@neuling.org>

Makefile

@@ -176,7 +176,7 @@ fpga_files = $(core_files) $(soc_files) fpga/soc_reset.vhdl \
 synth_files = $(core_files) $(soc_files) $(fpga_files) $(clkgen) $(toplevel) $(dmi_dtm)
 
 microwatt.json: $(synth_files) $(RAM_INIT_FILE)
-	$(YOSYS) -m $(GHDLSYNTH) -p "ghdl --std=08 --no-formal $(GHDL_IMAGE_GENERICS) $(GHDL_TARGET_GENERICS) $(synth_files) -e toplevel; synth_ecp5 -json $@" $(uart_files)
+	$(YOSYS) -m $(GHDLSYNTH) -p "ghdl --std=08 --no-formal $(GHDL_IMAGE_GENERICS) $(GHDL_TARGET_GENERICS) $(synth_files) -e toplevel; synth_ecp5 -json $@  $(SYNTH_ECP5_FLAGS)" $(uart_files)
 
 microwatt.v: $(synth_files) $(RAM_INIT_FILE)
 	$(YOSYS) -m $(GHDLSYNTH) -p "ghdl --std=08 --no-formal $(GHDL_IMAGE_GENERICS) $(GHDL_TARGET_GENERICS) $(synth_files) -e toplevel; write_verilog $@" $(uart_files)