Comment 7 for bug 20465

I believe that initramfs now moves /dev from the initramfs over to the installed
system, which means that udev will skip its initialization.

  start)
    if [ -e "$udev_root/.udevdb" ]; then
      if mountpoint -q /dev/; then
        log_failure_msg "FATAL: udev is already active on $udev_root."
        exit 1
      else
        log_warning_msg "WARNING: .udevdb already exists on the old $udev_root!"
     fi
    fi
    mount -n -o size=$tmpfs_size,mode=0755 -t tmpfs tmpfs $udev_root
    log_begin_msg "Creating initial device nodes..."
    udevstart
    log_end_msg $?

initramfs seems to create a file /dev/.initramfs-tools. Probably udev should
check for that, and if it exists, run udevstart (but not the mount).