ERROR: Module rst_gen contains processes, which are not supported by JSON backend (run `proc` first).

Edit Makefile

$(TARGET).json: $(OBJS)
#       yosys -p "synth_ecp5 -json $@" $(OBJS)
        yosys -p "read_verilog -sv $<" -p "synth_ecp5 -json $@" $(OBJS)