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

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>