ramips-mt7621: add support for D-Link COVR-X1860 a1

[maurerle]

The COVR-X1860 is a wifi6 device quite similar to the DAP-X1860, which is not wall-plugged and has two Gigabit LAN ports.
It is already well tested with ~40 devices in FFAC.

• Must be flashable from vendor firmware
  • Web interface (recovery when holding the reset button while booting or signed factory image when using D-Link 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())')
• 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 (none)
    • Should map to their respective radio
    • Should show activity
  • Switch port LEDs (none)
    • 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

[maurerle]

this PR currently awaits #3152 but can be merged afterwards

[blocktrron]

Please rebase on latest master.

[maurerle]

I just did build firmware from the rebase and tested it on one of my devices.
This is ready to merge