The factory images need to embed specific IDs to pass verification with
the OEM firmware (including TFTP recovery), so they need to be
per-device variables.
Fixes: ab1584a797 ("ath79: netgear: trim down uImage customisations")
Fixes: 459c8c9ef8 ("ath79: add support for ZyXEL NBG6616")
Reported-by: Marcin Juszkiewicz <marcin-openwrt@juszkiewicz.com.pl>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
[minor commit message adjustments, sort DEVICE_VARS]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>