mediatek: add support for JioRouter AX6000 JIDU6101

mediatek: add support for JioRouter AX6000 JIDU6101

| Component        | Details                                         |
|------------------|-------------------------------------------------|
| **SoC**          | MediaTek MT7986A (4× ARM Cortex-A53 @ 2.0 GHz) |
| **RAM**          | 512 MB                                          |
| **Flash**        | 256 MB NAND                                     |
| **Ethernet**     | 5× 10/100/1000 Mbps (1 WAN + 4 LAN)            |
| **WLAN 2.4 GHz** | MediaTek MT7976GN — 802.11b/g/n/ax, 4×4 MIMO   |
| **WLAN 5 GHz**   | MediaTek MT7976AN — 802.11n/ac/ax, 4×4 MIMO    |
| **LEDs**         | 1× RGB LED (GPIO-controlled)                    |
| **Button**       | 1× Reset                                        |
| **USB**          | Yes                                             |

**MAC Addresses:**

| Interface  | Source                                          |
|------------|-------------------------------------------------|
| WAN/Label  | u-boot-env MTD partition, "mac" (text)          |
| LAN        | WAN + 1                                         |
| 2.4 GHz    | WAN + 2                                         |
| 5 GHz      | WAN + 3                                         |

---

**1. Prepare TFTP server**

Set a static IP on the ethernet interface of your computer (e.g. default: ip `192.168.1.2`, gateway `192.168.1.1`).

Download the initramfs image and host it with the TFTP server.

**2. Interrupt boot**

Attach UART and power on the router. When the boot menu appears, select **Failsafe Mode**,
then press `Ctrl-C` to interrupt and enter the U-Boot prompt.

**3. Load and run initramfs image**
```sh
setenv ipaddr 192.168.1.1
setenv serverip 192.168.1.2
tftpboot 0x46000000 openwrt-mediatek-filogic-jiorouter_ax6000-jidu6101-initramfs-kernel.bin
fdt addr $(fdtcontroladdr)
fdt rm /signature
bootm
```

**4. Flash sysupgrade image**

Place the sysupgrade image in `/tmp`, then run:
```sh
sysupgrade /tmp/openwrt-mediatek-filogic-jiorouter_ax6000-jidu6101-squashfs-sysupgrade.bin
```
Alternatively, use the sysupgrade option in LuCI.

Note:
- The raw MTD u-boot-env partition is only used for MAC address storage, while the active U-Boot environment is stored in the UBI volume.
- These devices ship with secure boot enabled and stock U-Boot only accepts vendor-signed FIT images.
- BL2/FIP cannot be replaced, so the stock signature verification path cannot be disabled directly.
- Setting ipaddr='' forces U-Boot to exit the web failsafe path and continue into autoboot.
- The custom bootcmd loads OpenWrt from the UBI volume and removes the /signature node before bootm, allowing unsigned OpenWrt FIT images to boot.
- Stock U-Boot expects its environment in a UBI volume named u-boot-env, so it is created during initial setup.

Signed-off-by: sh3ikh-faisal <sheikhfaisal713@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22201
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>