systemd: Add mkinitcpio support to plymouth-switch-root-initramfs.service

systemd: Add mkinitcpio support to plymouth-switch-root-initramfs.service

plymouth-switch-root-initramfs.service commands plymouth to release
rootfs to allow the shutdown ramfs to unmount rootfs. However, it only
integrates with dracut. Arch Linux uses mkinitcpio by default, and it
uses shutdown ramfs when layered block storage (e.g. LUKS) is used for
rootfs. Therefore, it fails to unmount rootfs on shutdown if LUKS and
plymouth are used together.

Add a new trigger for plymouth-switch-root-initramfs.service to start
when /run/initramfs/shutdown exists (/run/initramfs/bin/sh may not exist
with mkinitcpio when systemd-based initramfs is used), and add a
dependency to start it after mkinitcpio generates the shutdown ramfs.