Store setup settings in eeprom

To have the values of `value_contrast` and `value_backlight` persist
over turn offs they are loaded and stored from and in the eeprom.

For preventing the flash target from earasing the eeprom the `EESAVE`
high fuse byte is set to 0 (programmed).

The values are now loaded when booted and stored when a new value is
set in the setup menu.

firmware/src/Makefile

@@ -5,9 +5,12 @@ PROGRAMMER := usbasp
 SPEED := 8000000UL
 
 TARGET := main.hex
+EEP := main.eep
 BIN := main.elf
 OBJ := main.o
 
+HIGH_FUSE := 0xD6
+
 SHELL := sh
 CC := avr-gcc
 OBJCOPY := avr-objcopy
@@ -16,24 +19,33 @@ AVRDUDE := avrdude
 
 CFLAGS := -mmcu=$(MCU) -D F_CPU=$(SPEED) -Os -Wall -Werror -Wextra
 
-all: $(TARGET)
+all: $(TARGET) $(EEP)
 
 $(TARGET): $(BIN)
 	${OBJCOPY} -O ihex -j .text -j .data $< $@
 
+$(EEP): $(BIN)
+	${OBJCOPY} --change-section-lma .eeprom=0 -O ihex -j .eeprom $< $@
+
 $(BIN): $(OBJ)
 	$(CC) $(CFLAGS) $< -o $@
 
 %.o: %.c
 	$(CC) $(CFLAGS) -c $< -o $@
 
-.PHONY: flash clean check size
+.PHONY: flash eeprom fuses clean check size
 
 flash: $(TARGET) size
 	$(AVRDUDE) -p $(MCU) -c $(PROGRAMMER) -U flash:w:$<:a
 
+eeprom: $(EEP) size
+	$(AVRDUDE) -p $(MCU) -c $(PROGRAMMER) -U eeprom:w:$<:a
+
+fuses:
+	$(AVRDUDE) -p $(MCU) -c $(PROGRAMMER) -U hfuse:w:$(HIGH_FUSE):m
+
 clean:
-	$(RM) $(TARGET) $(BIN) $(OBJ)
+	$(RM) $(TARGET) $(EEP) $(BIN) $(OBJ)
 
 check:
 	cppcheck main.c