scripts/icestorm: move SHIFT_COUNTER_BITS into Makefile

This makes it easier to build separate bitstreams for simulation targets
vs the real bitstream for hardware, without editing any source code.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

scripts/icestorm/Makefile

@@ -1,13 +1,19 @@
 TOOLCHAIN_PREFIX = riscv32-unknown-elf-
 
+# set to 4 for simulation
+FIRMWARE_COUNTER_BITS=18
+
 all: example.bin
 
 ## -------------------
 ## firmware generation
 
 firmware.elf: firmware.S firmware.c firmware.lds
-	$(TOOLCHAIN_PREFIX)gcc -march=rv32i -Os -ffreestanding -nostdlib -o $@ firmware.S firmware.c \
-			--std=gnu99 -Wl,-Bstatic,-T,firmware.lds,-Map,firmware.map,--strip-debug -lgcc
+	$(TOOLCHAIN_PREFIX)gcc \
+		-DSHIFT_COUNTER_BITS=$(FIRMWARE_COUNTER_BITS) \
+		-march=rv32i -Os -ffreestanding -nostdlib \
+		-o $@ firmware.S firmware.c \
+		--std=gnu99 -Wl,-Bstatic,-T,firmware.lds,-Map,firmware.map,--strip-debug -lgcc
 	chmod -x $@
 
 firmware.bin: firmware.elf

scripts/icestorm/firmware.c

@@ -1,7 +1,8 @@
 #include <stdint.h>
 
-// use SHIFT_COUNTER_BITS=4 for simulation
-#define SHIFT_COUNTER_BITS 18
+#ifndef SHIFT_COUNTER_BITS
+#error SHIFT_COUNTER_BITS must be defined as 4 (for simulation) or 18 (for hardware bitstreams)!
+#endif
 
 void output(uint8_t c)
 {