pmb.flasher/export: Add --no-install option to skip kernel/initfs update

At the moment, pmbootstrap updates the kernel and the initfs whenever using the flasher or export. This is useful, but sometimes you just want to boot exactly the same kernel several times. In that case, having to wait several seconds for the (redundant) update to complete is quite annoying.

Add a --no-install option that allows skipping the kernel/initfs update.