Leave the generation of initramfs during update in Ubuntu

During every dist-upgrade the initramfs is generated multiple times and it's a bit annoying, and could be saved until later as it only needs generating once.

This could be done by simply adding a switch to dpkg to record this trigger until later, then as the last task on "installing packages" update the initramfs file. On many machines this task adds probably around ~5-15 minutes depending on machine, so I think this would be beneficial.