[Buildroot] Finding the linux kernel headers version of a gcc cross toolchain

[Laurentiu-Cristian Duca]

Hello buildroot community,

  When choosing an external toolchain for Buildroot
we must specify the kernel headers version used to build the
corresponding toolchain.
Can you please tell me how to detect the kernel headers version of the
external toolchain?

Thank you
Kind regards
L-C Duca