Automate kernel rebuild on toolchain update

Current situation:

We have overlays in the core repo like the kernel, systemd and currently a temporary glibc overlay. If there are toolchain updates or something, we have to rebuild the kernels. We don't sync until someone is ready to trigger the builds.

What we would like is automation of that. Some first questions that need to be addressed are posed in #3 (comment 31473):

• How to determine if a toolchain needs update
• Split the sync for core and extra
  • core (rebuild core)?
  • extra (directly) to Edge?
• What steps
• Which packages