ath79-nand: (re)add ZyXEL NBG6716

[Djfe]

Gone due to
commit 45c84a1 ("ar71xx: drop target")

I want to test two things still:
Upgrade from Gluon v2021.1.x to master (very likely not working => I won't add a manifest alias that would allow autoupdates to work)
From the check list: All network ports (done)

Readding was possible thanks to @techtimo

• Must be flashable from vendor firmware
  • Web interface
  • TFTP
  • Other:
• Must support upgrade mechanism
  • Must have working sysupgrade
    • Must keep/forget configuration (sysupgrade [-n], firstboot)
  • Gluon profile name matches autoupdater image name
    (lua -e 'print(require("platform_info").get_image_name())') zyxel-nbg6716
• Reset/WPS/... button must return device into config mode
• Primary MAC address should match address on device label (or packaging)
  (https://gluon.readthedocs.io/en/latest/dev/hardware.html#hardware-support-in-packages)
  • When re-adding a device that was supported by an earlier version of Gluon, a
    factory reset must be performed before checking the primary MAC address, as
    the setting from the old version is not reset otherwise.
• Wired network
  • should support all network ports on the device
  • must have correct port assignment (WAN/LAN)
    • if there are multiple ports but no WAN port:
      • the PoE input should be WAN, all other ports LAN
      • otherwise the first port should be declared as WAN, all other ports LAN
• Wireless network (if applicable)
  • Association with AP must be possible on all radios
  • Association with 802.11s mesh must work on all radios
  • AP+mesh mode must work in parallel on all radios
• LED mapping
  • Power/system LED
    • Lit while the device is on
    • Should display config mode blink sequence
      (https://gluon.readthedocs.io/en/latest/features/configmode.html)
  • Radio LEDs
    • Should map to their respective radio
    • Should show activity
  • Switch port LEDs
    • Should map to their respective port (or switch, if only one led present)
    • Should show link state and activity
• Outdoor devices only:
  • Added board name to is_outdoor_device function in package/gluon-core/luasrc/usr/lib/lua/gluon/platform.lua
• Cellular devices only:
  • Added board name to is_cellular_device function in package/gluon-core/luasrc/usr/lib/lua/gluon/platform.lua
  • Added board name with modem setup function setup_ncm_qmi to package/gluon-core/luasrc/lib/gluon/upgrade/250-cellular
• Docs:
  • Added Device to docs/user/supported_devices.rst

[Djfe]

I removed this manifest alias again, because sysupgrades won't work according to the commit that readded support. Upgrades/reinstalls need to be done manually because they increased the size of the Kernel partition to 4M. Also UBI needs to be erased on 256M variants. ath79 only supports up to 128M of NAND

	manifest_aliases = {
		'nbg6716', -- Upgrade from OpenWrt 19.07
	},

Notes for sysupgrades from Gluon 2021 and earlier:
Please follow these instructions:
https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=99835e09992bb411519fda1f8d16a6d07df7f598

Autoupdater is incompatible, because the migration can't be done via sysupgrade. It requires flashing the partition using the mtd tool.
The device exists in several variants, ar71xx supported all of them. ath79 (Gluon 22) will only support variants with up to 128MB NAND flash.
If your Zyxel has 256MB of NAND flash, then this image should still work but you need to erase your ubi partition first when migrating. (This might require overwriting the mtd partition with zeroes but I don't know. I have no device to test. Maybe the command in the git commit is enough already. In-case of failure you can still try recovering using tftp, see the OpenWrt wiki for instructions)
Else ubi might cause bootloops when the existing partition on flash doesn't end within the limits set by mtd offset and size.