令人迷惑的Makefile
產出module ko, 有2種方式
kernel build, menuconfig select M
Single build.

---

若,single buid， make指令有帶4個參數
CROSS_COMPILE=
ARCH=
-C "..."
M="..."

Makefile內容,只要
obj-m:=demo.o

狀況類似 kernel build with M.

-----------------
若,single build，make指令不帶參數
Makefile內容,就要複雜點了
make前要export ARCH 及 CROSS_COMPILE

---

ifneq ($(KERNELRELEASE),)
obj-m:=demo.o
else
KSRC="/home/ai7688/0221/openwrt-b4d7885/build_dir/target-mipsel_24kc_musl/linux-ramips_mt76x8/linux-5.4.154"

modules:
$(MAKE) ARCH=$(ARCH) CROSS_COMPILE=$(CROSS_COMPILE) -C $(KSRC) M=$(shell pwd) modules

clean:
rm -rf *.o *.ko *~ *.mod.c *.order *.symvers

endif