Currently the kernel-install man page only documents the bls layout for use
with the boot loader spec type #1. 90-loaderentry.install uses this layout to
generate loader entries and copy the kernel image and initrd to $BOOT.
This commit documents a second layout "uki" and adds 90-uki-copy.install,
which copies a UKI "uki.efi" from the staging area or any file with the .efi
extension given on the command line to
$BOOT/EFI/Linux/$ENTRY_TOKEN-$KERNEl_VERSION(+$TRIES).efi
This allows for both locally generated and distro-provided UKIs to be handled
by kernel-install.