Replace "with_up" by "with_stock"

This work started in kernel-ark!4225 by @ahajkova

The term 'up' used to refer to the specific kernel for a uni-processor. Today, this term 'up' remains in the spec file but it no longer retains any of its original meaning. Currently 'up' refers to the stock kernel build that produces the kernel, kernel-core, kernel-modules, etc. packages. I propose renaming with_up variable to with_stock to avoid possible confusion and make reading the spec file easier.