Use makefile includes

Makefile

@@ -51,19 +51,11 @@ all: kern/kernel.elf
 quickstart:
 	make get_toolchain && bear -- make -j$(nproc) && make qemu
 
-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/libkern/stdio.o \
-	kern/libkern/badrand.o
+SUBDIRS := kern
+
+include $(patsubst %, %/include.mk, $(SUBDIRS))
+
+KERNEL_OBJ := $(KERN_OBJ)
 
 kern/kernel.elf: $(KERNEL_OBJ)
 	@echo LD $@