Refactor optimization flag in Makefile

Introduce `O_FLAG` variable for `-Os` to centralize optimization setting
for CFLAGS and LDFLAGS.

firmware/atmega/Makefile

@@ -8,15 +8,16 @@ LD = $(CC)
 OC = avr-objcopy
 RM = rm -f
 
+O_FLAG = -Os
+
 BUILD_DIR = build
 OBJ_DIR = $(BUILD_DIR)/obj
 
 SRCS_C = source/adc.c source/moisture.c source/main.c source/pump.c
-
 OBJS = $(addprefix $(OBJ_DIR)/, $(notdir $(SRCS_C:.c=.o)))
 
-CFLAGS = -Wall -Os -DF_CPU=$(F_CPU) -mmcu=$(MCU) -Iinclude
-LDFLAGS = -Wall -Os -mmcu=$(MCU)
+CFLAGS = -Wall $(O_FLAG) -DF_CPU=$(F_CPU) -mmcu=$(MCU) -Iinclude
+LDFLAGS = -Wall $(O_FLAG) -mmcu=$(MCU)
 LIBS = -lm
 
 EXEC = atmega