initrd: add unit files needed for basic systemd-in-initrd support

This will:
 * mount all configured filesystems (typically the rootfs on /sysroot)
 * reload the configuration to pick up anything from the mounted fs (typically
   /sysroot/etc/fstab)
 * mount any newly configured filesystems (typically /usr on /sysroot/usr, if
   applicable)
 * shut-down and clean-up any daemons running in the initramfs (typically udevd)
 * switch-root to /sysroot and start the real init

For an example of what files should be included in an initramfs based on this
see
<https://mailman.archlinux.org/pipermail/arch-projects/2013-February/003628.html>.

Cc: Harald Hoyer <harald.hoyer@gmail.com>
Cc: Dave Reisner <d@falconindy.com>

Makefile.am

@@ -379,7 +379,12 @@ dist_systemunit_DATA = \
 	units/systemd-ask-password-console.path \
 	units/systemd-udevd-control.socket \
 	units/systemd-udevd-kernel.socket \
-	units/system-update.target
+	units/system-update.target \
+	units/initrd-parse-etc.service \
+	units/initrd-cleanup.service \
+	units/initrd-switch-root.target \
+	units/initrd-udevadm-cleanup-db.service \
+	units/initrd-switch-root.service
 
 nodist_systemunit_DATA = \
 	units/getty@.service \

units/initrd-cleanup.service

@@ -0,0 +1,18 @@
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Cleaning Up and Shutting Down Daemons
+DefaultDependencies=no
+ConditionPathExists=/etc/initrd-release
+OnFailure=emergency.target
+Requires=local-fs.target swap.target
+After=local-fs.target swap.target
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/systemctl --no-block isolate initrd-switch-root.target

units/initrd-parse-etc.service

@@ -0,0 +1,19 @@
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Reload Configuration from the Real Root
+DefaultDependencies=no
+Requires=local-fs.target swap.target
+After=local-fs.target swap.target
+OnFailure=emergency.target
+ConditionPathExists=/etc/initrd-release
+
+[Service]
+Type=oneshot
+ExecStartPre=/usr/bin/systemctl daemon-reload
+ExecStart=/usr/bin/systemctl --no-block start initrd-cleanup.service

units/initrd-switch-root.service

@@ -0,0 +1,19 @@
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Switch Root
+DefaultDependencies=no
+ConditionPathExists=/etc/initrd-release
+OnFailure=emergency.target
+AllowIsolate=yes
+
+[Service]
+Type=oneshot
+# we have to use "--force" here, otherwise systemd would umount /run
+ExecStart=/usr/bin/systemctl --no-block --force switch-root /sysroot
+KillMode=none

units/initrd-switch-root.target

@@ -0,0 +1,16 @@
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Switch Root
+ConditionPathExists=/etc/initrd-release
+DefaultDependencies=no
+Requires=initrd-switch-root.service
+Before=initrd-switch-root.service
+AllowIsolate=yes
+Wants=initrd-udevadm-cleanup-db.service local-fs.target swap.target systemd-journald.service
+After=initrd-udevadm-cleanup-db.service local-fs.target swap.target emergency.service emergency.target

units/initrd-udevadm-cleanup-db.service

@@ -0,0 +1,18 @@
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Cleanup udevd DB
+DefaultDependencies=no
+ConditionPathExists=/etc/initrd-release
+Conflicts=systemd-udevd.service systemd-udevd-control.socket systemd-udevd-kernel.socket
+After=systemd-udevd.service systemd-udevd-control.socket systemd-udevd-kernel.socket
+Before=initrd-switch-root.target
+
+[Service]
+Type=oneshot
+ExecStart=-/usr/bin/udevadm info --cleanup-db