Kernel upgrading

The ayufan builds are released quite frequently.

Upgrading to a newer kernel (ex: kernel 4.15) is quite simple:

Uncomment pre-releases from /etc/apt/sources.list.d/ayufan-rock64.list
Search for available kernels: apt-cache search linux-image
Manually install the <kernel>, ex: linux-image-4.15.0-rockchip-ayufan-177-g59389fa34
```
apt-get update
apt-get install <kernel>
```
Reboot