Add support for mkinitcpio

https://bugzilla.gnome.org/show_bug.cgi?id=710682

src/boot/mkinitcpio/ostree

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+build() {
+    add_binary /usr/sbin/ostree-prepare-root
+    add_binary /usr/sbin/ostree-remount
+
+    add_file /usr/lib/systemd/system/ostree-prepare-root.service
+    add_symlink /usr/lib/systemd/system/initrd-switch-root.target.wants/ostree-prepare-root.service \
+        /usr/lib/systemd/system/ostree-prepare-root.service
+}

src/boot/mkinitcpio/ostree-mkinitcpio.conf

@@ -0,0 +1 @@
+HOOKS="base systemd ostree autodetect modconf block filesystems keyboard fsck"