Allow system.devices to be automatically created on first boot

Allow system.devices to be automatically created on first boot

An OS installer can create system.devices for the system and
disks, but an OS image cannot create the system-specific
system.devices.  The OS image can instead configure the
image so that lvm will create system.devices on first boot.

Image preparation steps to enable auto creation of system.devices:
- create empty file /etc/lvm/devices/auto-import-rootvg
- remove any existing /etc/lvm/devices/system.devices
- enable lvm-devices-import.path
- enable lvm-devices-import.service

On first boot of the prepared image:
- udev triggers vgchange -aay --autoactivation event <rootvg>
- vgchange activates LVs in the root VG
- vgchange finds the file /etc/lvm/devices/auto-import-rootvg,
  and no /etc/lvm/devices/system.devices, so it creates
  /run/lvm/lvm-devices-import
- lvm-devices-import.path is run when /run/lvm/lvm-devices-import
  appears, and triggers lvm-devices-import.service
- lvm-devices-import.service runs vgimportdevices --rootvg --auto
- vgimportdevices finds /etc/lvm/devices/auto-import-rootvg,
  and no system.devices, so it creates system.devices containing
  PVs in the root VG, and removes /etc/lvm/devices/auto-import-rootvg
  and /run/lvm/lvm-devices-import

Run directly, vgimportdevices --rootvg (without --auto), will create
a new system.devices for the root VG, or will add devices for the
root VG to an existing system.devices.