Here is the procedure to reproduce the Starter Package for this
st_stm32mp135f_dk_demo Buildroot configuration:

1. Clone the repositories:

   git clone git@github.com:bootlin/buildroot
   git -C buildroot checkout st/2023.02.10
   git clone git@github.com:bootlin/buildroot-external-st.git
   git -C buildroot-external-st checkout openstlinux-6.1-buildroot-2023.02.10-mp1-v23.11.15

2. Configure Buildroot

   make -C buildroot BR2_EXTERNAL=../buildroot-external-st O=../output st_stm32mp135f_dk_demo_defconfig

3. Build the st_stm32mp135f_dk_demo image

   make -C buildroot O=../output

4. Generate license compliance information

   make -C buildroot O=../output legal-info

5. Build the SDK

   make -C buildroot O=../output sdk

You will find the st_stm32mp135f_dk_demo image and sdk in:

  buildroot/output/images

You will find the st_stm32mp135f_dk_demo licenses and sources in:

  buildroot/output/legal-info