kernel-install .install scripts should support the UKI layout mode

The .install scripts by mkinitcpio should generate to /<esp>/EFI/Linux/<name>{,-fallback}.efi uki files only, if the esp layout is configured to UKI mode (maybe look for the layout option in /etc/kernel/install.conf?).

systemd's kernel-install tool runs .install scripts provided by mkinicpio during kernel installations.

Arch should have an easy way to configure kernel-install scripts to install UKI images to the expected /<esp>/EFI/Linux directory. Systemd kernel-install already has a layout configuration option passed to the .install scripts, mkinitcpio .install script could take that into account.

Also mkinitcpio preset generation should be configurable to generate presets for UKI vs the traditional images format.