See original article by Thomas Petazzoni at bootlin.com

Important - Preparing Buildroot

For this tutorial to work as outlined in the text below we need to set up a new copy of buildroot and switch to a specific earlier version buildroot it. There's 
a strong dependence between buildroot, the linux kernel and kernel additions and modifications by ST Microelectronics. On top there's also 
a dependency regarding the build tools involved (gcc in particular) on the build host. In case of the current version of Lab Assignment 3 we need
a gcc version not newer than nine (gcc-9). Otherwise we'll run into problems when buildroot tries to compile the device tree parser.

• make sure your on a host with gcc-9 compiler (eda1, zaphod)
• consider using multiple host / spreading across servers - the build process will take a lot of time
• zaphod is faster than eda1 and has more memory

an earlier version of it.
Create a new directory and clone the buildroot repository and switch to version 2020.05.3:

mkdir lab3
cd lab3
git clone git://git.buildroot.net/buildroot
cd buildroot
git checkout -b bme280 2020.05.3
make stm32mp157a_dk1_defconfig
make menuconfig
make

...