Provide kernel build environments for older kernel version

[0xricksanchez]

As it is now only "recent" kernel version can be compiled. I did not check yet how far back we can go currently but it would be nice to have build environments for at least the major kernel releases eventually.

[goldstar611]

Buildroot can help with this too. I saw another issue that looks like you're interested in replacing debootstrap with buildroot. The buildroot option is BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE and BR2_GCC_VERSION_ to set a lower gcc version for earlier kernels.