ath79: fix factory image generation for Netgear and Zyxel boards

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: ab1584a797ad ("ath79: netgear: trim down uImage customisations")
Fixes: 459c8c9ef816 ("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>