econet: en751221: add support for Huawei HG2821T-U

econet: en751221: add support for Huawei HG2821T-U

This patch adds support for Huawei HG2821T-U, EPON ONU and home router,
often comes with ISP service.

Specifications
==============
* SoC: Econet EN7526GT
* RAM: 512MiB DDR3L (MT41K256M16TW-107)
* Flash: 256MiB SPI NAND (TC58CVG1S3HRAIG)
* WiFi 2.4GHz: MT7592N
* WiFi 5GHz: MT7612EN
* Ethernet: builtin switch
* LED: Power, Internet, WiFi, USB
* Buttons: Reset
* UART: Serial console (115200 8n1)
* USB: 1 x USB2
* Other: Phone jack, XPON fiber port

!!! BACKUP YOUR ROM !!!
=======================
Please always have your FULL flash image backup before flashing
anything. The vendor firmware varies a lot depending on your ISP and
location. You will have a hard time finding the right regional firmware
if you don't have a backup.

Notes
=====
* Due to the target `econet` being incomplete, WiFi, DSA switch, and
many other features are not supported yet. Do not flash the image unless
you know the consquences or `econet` is declared stable.
* This device, and apparently many other devices of this platform, use
a dual-image layout. OpenWRT (with `econet` target) only uses slot A.
Slot B is not used by OpenWRT, and is applicable for dual-booting to
vendor firmware.
* If you do not use vendor firmware anymore, you can erase and reuse
anything after `configuration_b`, which gives you ~110 MiB free space.
Again, backup your flash first.

Installation
============
Within shell
------------
Note that acquiring the shell access to the vendor firmware can be a bit
tricky depending on the firmware variation. If you can't play with the
vendor firmware, boot to OpenWrt using debricking method below.

0. (Optional) Back up your flash, and / or move the vendor firmware to
   slot B
1. Build and then locate the `kernel.bin` and `rootfs.bin` image files
2. Upload `kernel.bin` and `rootfs.bin` to the device (via HTTP or USB
   stick), then type:
   ```
   mtd write -f -e KernelA kernel.bin KernelA
   mtd erase RootfsA
   mtd write -f -r -e AppA rootfs.bin AppA
   ```

From bootloader
---------------
1. Build and then locate the `kernel.bin` and `rootfs.bin` image files
2. Switch device on and press a key within 3 seconds
3. Upload `kernel.bin` via TFTP as described below
4. Once the transfer has completed successfully, bootloader will give
   you the file length in "Total %d (0x%X) bytes received", then type
   `flash 200000 80020000 <file length hex>`
5. Upload `rootfs.bin` then flash with
   `flash 600000 80020000 <file length hex>`
6. Restart the device to boot into OpenWRT

> [!IMPORTANT]
> Do not try `httpd` in the bootloader. It writes to the wrong address
  and will corrupt the flash.

Debricking
==========
1. Build and then locate the `initramfs-kernel.bin` image files
2. Switch device on and press a key within 3 seconds
3. Connect to device via ethernet, set the IP address to `192.168.1.X`,
   then upload the image via TFTP
   `tftp 192.168.1.1 -m binary -v -c put initramfs-kernel.bin`

   The file name can be anything except `tcboot.bin` or `tclinux.bin`,
   they will corrupt the flash.
4. Type `jump 80020000` to boot the kernel from memory

Dual boot
=========
Use `en75_chboot` tool to switch between vendor firmware and OpenWrt. If
you are soft-locked, you can also switch the flag in the bootloader:
1. Switch device on and press a key within 3 seconds
2. Select the kernel that you wish to use:
   - `memwl 80020000 30ffffff` for `KernelA` (OpenWrt)
   - `memwl 80020000 31ffffff` for `KernelB` (Factory)
3. Select the rootfs, which should be the same as the kernel:
   - `memwl 80020004 30ffffff` for `RootfsA` (OpenWrt)
   - `memwl 80020004 31ffffff` for `RootfsB` (Factory)
3. Commit the data to flash: `flash 1e0000 80020000 8`
4. Restart the device to boot into the selected OS

MAC addresses
=============
`//configuration_a/factory.conf` contains MAC addresses, along with
other pre-configured settings. OpenWrt uses `brmac`, `internetmac`,
`APMAC`, and `APMAC_5G`, while `tr069mac`, `voipmac`, `priprotocolmac`,
and `PONMac` are not used for now.

Signed-off-by: David Yang <mmyangfl@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23131
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>