Make base kernel packages save the old kernel on deinstallation

[madpilot]

Summary

As the subject states I've made this modification to force the kernel base packages to save a copy of the kernel directory in kernel.old.

I'm doing it through deinstall script. The script I added actually mimics what the Makefiles do when installkernel is called.

Not sure this is the best solution, maybe it partly beats the point of using packages to manage kernels, but I think it important to preserve a copy of the previous kernel when upgrading to have a fallback in case of problems.

Test Plan

I've been using this on my machines for a while with no ill effects.

[madpilot]

Better plan is in the works.