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)

target/linux/mediatek/image/filogic.mk

@@ -728,6 +728,23 @@ endif
 endef
 TARGET_DEVICES += xiaomi_redmi-router-ax6000-ubootmod
 
+define Device/zbtlink_zbt-z8102ax
+  DEVICE_VENDOR := Zbtlink
+  DEVICE_MODEL := ZBT-Z8102AX
+  DEVICE_DTS := mt7981b-zbtlink-zbt-z8102ax
+  DEVICE_DTS_DIR := ../dts
+  DEVICE_PACKAGES := kmod-mt7981-firmware mt7981-wo-firmware kmod-usb-net-qmi-wwan kmod-usb-serial-option
+  KERNEL_IN_UBI := 1
+  UBINIZE_OPTS := -E 5
+  BLOCKSIZE := 128k
+  PAGESIZE := 2048
+  IMAGE_SIZE := 65536k
+  IMAGES += factory.bin
+  IMAGE/factory.bin := append-ubi | check-size $$(IMAGE_SIZE)
+  IMAGE/sysupgrade.bin := sysupgrade-tar | append-metadata
+endef
+TARGET_DEVICES += zbtlink_zbt-z8102ax
+
 define Device/zyxel_ex5601-t0-stock
   DEVICE_VENDOR := Zyxel
   DEVICE_MODEL := EX5601-T0  (stock layout)