feat(install.d): allow using dracut in combination with ukify

feat(install.d): allow using dracut in combination with ukify

This can be simply accomplished with the KERNEL_INSTALL_INITRD_GENERATOR
and KERNEL_INSTALL_UKI_GENERATOR variables. `60-ukify.install` looks for
the initrd in the KERNEL_INSTALL_STAGING_AREA and then takes care of building
the uki.

This change makes it possible to use a configuration like this:
/etc/kernel/install.conf
layout=uki
uki_generator=ukify
initrd_generator=dracut

Without this change this configuration will fail since dracut would also
generate an uki instead of initrd, which will cause a problem in
`60-ukify.install` since it can't find an initrd to use.

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