Add documentation on updating kernel config file

[razvand]

There is no documentation on how to use and update the tools/labs/qemu/kernel_config.x86 file. Document the use of make olddefconfig and make savedefconfig.

[scotthuang1989]

Is there update on this issue? I have same issue of how to use tools/labs/qemu/kernel_config.x86.
I have to manually select configuration when I first run make boot