SM8550 kernel compile

hi
here i follow the doc from this link
https://devboardsforandroid.linaro.org/en/latest/devices/sm8550-hdk.html#getting-started-with-sm8550-hdk
1 compile the android part successfully, but flash the sm8550 develop board can not boot up

2 download the kernel code
git clone Linaro Git Hosting -b rbX-mainline
but do not found this config rbX_aosp_defconfig in this branch rbX-mainline

could you help me check?

thanks