mediatek: add support for Zbtlink ZBT-Z8102AX

Specifications:
SoC: MediaTek MT7981B
RAM: 1024MiB
Flash: SPI-NAND 128 MiB
Switch: 1 WAN, 4 LAN (Gigabit)
USB: two M.2 slots for 5G modems via USB 3.0 hub, external USB 3.0 port
Buttons: Reset, Mesh
Power: DC 12V 1A
WiFi: MT7976CN
UART: 115200n8
UART Layout:
VCC-RX-TX-GND

Installation:
A. Through OpenWrt Dashboard:
If your router comes with OpenWrt preinstalled (modified by the seller),
you can easily upgrade by going to the dashboard (192.168.1.1) and then
navigate to System -> Backup/Flash firmware, then flash the firmware

B. Through TFTP
Standard installation via UART:

1. Connect USB Serial Adapter to the UART, (NOTE: Don't connect the VCC pin).
2. Power on the router. Make sure that you can access your router via UART.
3. Restart the router then repeatedly press ctrl + c to skip default boot.
4. Type > bootmenu
5. Press '2' to select upgrade firmware
6. Press 'Y' on 'Run image after upgrading?'
7. Press '0' and hit 'enter' to select TFTP client (default)
8. Fill the U-Boot's IP address and TFTP server's IP address.
9. Finally, enter the 'firmware' filename.

Based on patch adding support for similar Zbtlink ZBT-Z8103AX device by
Ian Ishmael C. Oderon.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit c8c2f52262)

package/boot/uboot-envtools/files/mediatek_filogic

@@ -66,7 +66,8 @@ mercusys,mr90x-v1)
 	ubootenv_add_uci_config "$envdev" "0x0" "0x20000" "0x20000" "1"
 	;;
 cetron,ct3003|\
-netgear,wax220)
+netgear,wax220|\
+zbtlink,zbt-z8102ax)
 	ubootenv_add_uci_config "/dev/mtd1" "0x0" "0x20000" "0x20000"
 	;;
 ubnt,unifi-6-plus)