fix(install.d): respect more kernel-install env variables

fix(install.d): respect more kernel-install env variables

- If kernel-install has defined a staging area for us
(KERNEL_INSTALL_STAGING_AREA) install generated initrd/uki.efi there.
The actual install is then handled by 90-loaderentry.install or
90-uki-copy-install.

- Also skip regeneration if an uki.efi already exists.

- Pass --kernel-image to dracut, this is required to generate an uki (uefi=yes)

- Add --no-uefi argument to dracut rescue image generation, this ensures that
it at least installs correctly. TODO: Rework 51-dracut-rescue.install to also
work with uki's.

This fixes installing a kernel with uefi=yes in dracut config and layout=uki
in kernel/install.conf.

Signed-off-by: Andrew Ammerlaan <andrewammerlaan@gentoo.org>