Add ability to generate/install initrd or unified kernel image

[Nowa-Ammerlaan]

This basically copies the code from dist-kernel-utils.eclass. Doing this here instead gives several advantages:

• Aligned behaviour with installkernel-systemd, which also generates an initrd or uki with dracut if it is installed.
• Aligned behaviour between manually compiled kernels (i.e. gentoo-sources) and dist-kernels (i.e. gentoo-kernel). With both it is now possible to automatically generate and install an initrd or uki if dracut is installed.
• Allows to significantly simplify dist-kernel-utils.eclass and kernel-install.eclass. Code paths that are now required only for installkernel-gentoo, but not for installkernel-systemd can be removed.

What is new is the ability to specify an alternate initrd generator, and the ability to pass additional arguments. Examples of useful arguments one might want to pass are: --verbose or --uefi/--no-uefi (to toggle between generating an initrd or uki).

CC @desultory @gentoo/dist-kernel @mgorny See-also: https://github.com/gentoo/gentoo/pull/33727

[Nowa-Ammerlaan]

@mgorny After the discussion today on IRC I have changed this to only use dracut (or another initrd generator) if explicitly requested, and adjusted the eclass patch at https://github.com/gentoo/gentoo/pull/33727 to explicitly request dracut.

Now there is no functionally different behaviour (by default) for both installkernel-gentoo and installkernel-systemd.

[Nowa-Ammerlaan]

Gentle ping, I now also added an updated ebuild for sys-kernel/installkernel-gentoo-8 over at https://github.com/gentoo/gentoo/pull/33727