Allow update-initramfs to run normally during package upgrade and installation

pkgsel's diversion of update-initramfs during package upgrade and
installation means that if a new kernel package is installed due to an
ABI bump, it won't have an initramfs image.

Running "update-initramfs -u" afterwards doesn't help, because that
only updates the most recent initramfs image that's already present.
For a new kernel package, we would need to run
"update-initramfs -c -k <abi-version>".

Also, during package installation "update-initramfs -u" normally only
activates a trigger, so updates will be coalesced anyway.

Instead of making this hack more complicated, remove it:

* Don't divert update-initramfs
* Move the setup code needed for update-initramfs before the first upgrade
* Move the cleanup code to the cleanup function

Closes: #912073