Branch out all kernel objects into a KERNEL_OBJ variable

Makefile

@@ -31,9 +31,19 @@ CFLAGS += -I./kern/libkern
 
 all: kern/kernel.elf
 
-main: main.o
+KERNEL_OBJ := \
+	kern/entry.o \
+	kern/start.o \
+	kern/kalloc.o \
+	kern/libkern/string.o \
+	kern/libkern/proc.o \
+	kern/libkern/uart.o \
+	kern/libkern/panic.o \
+	kern/libkern/memory.o \
+	kern/libkern/spinlock.o \
+	kern/libkern/mini-printf.o
 
-kern/kernel.elf: kern/entry.o kern/start.o kern/libkern/string.o kern/libkern/proc.o kern/libkern/uart.o kern/libkern/panic.o kern/kalloc.o kern/libkern/memory.o kern/libkern/spinlock.o kern/libkern/string.o kern/libkern/mini-printf.o
+kern/kernel.elf: $(KERNEL_OBJ)
 	@echo LD $@
 	@$(LD) $(LDFLAGS) -o $@ $^