Only build files that have changed

Makefile

@@ -4,21 +4,20 @@ FILES = src/int21.s \
         src/kernel.s
 
 .PHONY: all qemu clean
-all: prepare boot kernel img
+all: obj/boot.o obj/kernel.o $(TARGET)
 
 qemu: all
 	qemu-system-i386 -fda $(TARGET)
 
-prepare:
-	mkdir -p obj
-
-boot: src/boot.s
+obj/boot.o: src/boot.s
+	@mkdir -p obj
 	nasm -I. -Isrc src/boot.s -o obj/boot.o
 
-kernel: $(FILES)
+obj/kernel.o: $(FILES)
+	@mkdir -p obj
 	nasm -I. -Isrc src/kernel.s -o obj/kernel.o
 
-img:
+$(TARGET):
 	cat obj/boot.o obj/kernel.o > $(TARGET)
 
 clean: