Add instructions for compiling a kernel module

recipes/kernel-module-compilation.md

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+# Kernel module compilation
+
+Follow these instructions to compile an additional kernel module.
+
+In this example, we'll compile the `ds1307 RTC` kernel module for version `4.4.154-1124`:
+
+1. Obtain your kernel version: `uname -r`
+    (ex: `4.4.154-1124-rockchip-ayufan-ged3ce4d15ec1`)
+2. Download the kernel source code which matches your kernel version from [github.com/ayufan-rock64/linux-kernel/releases](https://github.com/ayufan-rock64/linux-kernel/releases)
+3. Extract the kernel: `tar -zxvf 4.4.154-1124-rockchip-ayufan.tar.gz`
+4. Copy the old kernel config:
+    ```bash
+    cd linux-kernel-4.4.154-1124-rockchip-ayufan
+    cp /usr/src/linux-headers-4.4.154-1124-rockchip-ayufan-ged3ce4d15ec1/.config .
+    make oldconfig
+    ```
+5. Install required build packages: `apt-get install bc python libncurses5-dev`
+6. Configure the kernel with the additional modules you want: `make menuconfig`
+    (ex: `Device Drivers --> [*] Real Time Clock --> <M>   Dallas/Maxim DS1307..`)
+7. Exit and save the config
+8. Prepare the modules:
+    ```bash
+    make EXTRAVERSION=-1124-rockchip-ayufan-ged3ce4d15ec1 modules_prepare
+    ````
+9. Build the module: `make M=drivers/rtc`
+    (note: specify the correct directory containing the module you added)
+10. Copy the module to the modules directory:
+    ```bash
+    mkdir -p /lib/modules/4.4.154-1124-rockchip-ayufan-ged3ce4d15ec1/kernel/drivers/rtc/
+    cp drivers/rtc/rtc-ds1307.ko /lib/modules/4.4.154-1124-rockchip-ayufan-ged3ce4d15ec1/kernel/drivers/rtc/
+    ```
+11. Ensure module is added to dep file:
+    ```bash
+    echo 'kernel/drivers/rtc/rtc-ds1307.ko:' >> /lib/modules/4.4.154-1124-rockchip-ayufan-ged3ce4d15ec1/modules.dep
+    ```
+12. Load the module:
+    ```bash
+    depmod -a
+    modprobe rtc-ds1307
+    ```
+13. Ensure module is loaded on boot: `echo "rtc-ds1307" >> /etc/modules`