To build and use the buildroot stuff, do the following:

1) make stm32mp157_dk_defconfig
2) make menuconfig 
    select the packages you wish to compile
3) make 
    wait while it compiles
find the kernel, bootloader, root filesystem, etc. in output/images



Buildroot is a simple, efficient and easy-to-use tool to generate embedded
Linux systems through cross-compilation.

The documentation can be found in docs/manual. You can generate a text
document with 'make manual-text' and read output/docs/manual/manual.text.
Online documentation can be found at http://buildroot.org/docs.html